Task Workspace Research Articles

Robot base placement and tool mounting optimization based on capability map for robot-assistant camera holder

AbstractIn the field of laparoscopic surgery, research is currently focusing on the development of new robotic systems to assist practitioners in complex operations, improving the precision of their medical gestures. In this context, the performance of these robotic platforms can be conditioned by various factors, such as the robot’s accessibility and dexterity in the task workspace. In this paper, we present a new strategy for improving the kinematic and dynamic performance of a 7-degrees of freedom robot-assisted camera-holder system for laparoscopic surgery. This approach involves the simultaneous optimization of the robot base placement and the laparoscope mounting orientation. To do so, a general robot capability representation approach is implemented in an innovative multiobjective optimization algorithm. The obtained results are first evaluated in simulation and then validated experimentally by comparing the robot’s performances implementing both the existing and the optimized solution. The optimization result led to a 2% improvement in the accessibility index and a 14% enhancement in manipulability. Furthermore, the dynamic performance criteria resulted in a substantial 43% reduction in power consumption.

Complete Kinematics/Dynamics Modeling and Performance Analysis of a Novel SCARA Parallel Manipulator Based on Screw Theory

Abstract In this paper, a novel Selective Compliance Assembly Robot Arm (SCARA) high-speed parallel manipulator that can realize three-translation and one-rotation motion is proposed, and an accurate dynamic modeling methodology is investigated. The mechanism is composed of four limbs with a double parallelogram structure and a single moving platform. The high bearing capacity and high dynamic response of the novel mechanism make it a viable alternative choice for this kind of automation equipment. The degree-of-freedom (DOF) of the mechanism is analyzed by the screw theory. At the same time, the velocity mapping model of the mechanism is established by the twist screw and the actuated Jacobian matrix. Then, the acceleration mapping model of the mechanism, including the generalized kinematic pairs, is established by reduced acceleration state, the modified Lie screw, and the acceleration Hessian matrix. On this basis, the complete dynamic model with a compact form of the mechanism is deduced by the combination of screw theory and virtual work principle, and the correctness of the developed model is verified by multibody simulation software. Finally, considering the inertial characteristics of the mechanism, the dynamic performance distribution in the reachable workspace of the mechanism is analyzed by the Joint-Reflected Inertia (JRI) index and Coefficient of Variation of joint-space Inertia (CVI) index, and some areas are selected as the task workspace using the above index to guarantee good dynamic performance.

Kinematics, dynamics and multi-objective optimization based on singularity-free task workspace for a novel SCARA parallel manipulator

Kinematics, dynamics and multi-objective optimization based on singularity-free task workspace for a novel SCARA parallel manipulator

Development of a novel hybrid haptic (nHH) device with a remote center of rotation dedicated to laparoscopic surgery

AbstractThis paper focuses on developing a novel hybrid-haptic (nHH) device with a remote center of rotation with 4 DOFs (degrees of freedom) intendant to be used as a haptic device. The new architecture is composed of two chains handling each one a part of the motions. It has the advantages of a parallel robot as high stiffness and accuracy, and the large workspace of the serial robots. The optimal synthesis of the nHH was performed using real-coded genetic algorithms. The optimization criteria and constraints were established and successively formulated and solved using a mono-objective function. A validation and comparison study were performed between the spherical parallel manipulator and the nHH. The obtained results are promising since the nHH is compared to other similar task devices, such as spherical parallel manipulator, and presents a suitable kinematic performance with a task workspace free singularity inside.

Fuzzy gain scheduling PID control of a hybrid robot based on dynamic characteristics

In this paper, a fuzzy gain scheduling proportional-integral-differential (PID) controller based on dynamic characteristics of a hybrid robot named TriMule is presented. Since the equivalent inertia and gravity imposed on motor shafts vary with the robot configuration, it is difficult to obtain satisfactory control performance with a linear, fixed gain PID controller. After analyzing the effect mechanism of dynamic characteristics on control performance, the clustering analysis algorithm and fuzzy logic are combined to deduce the control parameters online according to the command acceleration, equivalent inertia and gravity. The experimental results on a prototype machine show that, compared with the conventional PID and fuzzy gain scheduling PID controller, the proposed method can significantly reduce the effect of dynamic characteristics on control system, thus ensuring satisfactory control performance under any configuration in the task workspace.

Kinematics of a 5-axis hybrid robot near singular configurations

• A method based on kinematics modification for singularity avoidance is proposed. • A criterion for choosing the appropriate inverse kinematic solution is presented. • A semi-analytical algorithm for forward kinematic analysis is developed. • An algorithm is developed by modifying C -axis instructions. • Both simulations and experiments are carried out to verify the proposed approach. This paper proposes a method for non-singular tool path generation based on inverse kinematics modification in joint space to deal with the singularity problem of a 5-axis hybrid robot in machining process. Firstly, a criterion for choosing the appropriate solution of inverse displacement analysis is presented, that enables the avoidance of mechanical interference and enlargement of the task workspace. As a counterpart of inverse displacement analysis, a semi-analytical algorithm for forward displacement analysis is developed with high computational efficiency. These works lay a comprehensive and solid foundation for motion control and tool path simulation of the hybrid robot. Then, it reveals that at a singular configuration that varies with the system configurations, sudden changes occur in both rotation of the C -axis of the 2-DOF wrist and lengths of three telescopic legs within the 3-DOF parallel mechanism. This finding leads to an effective method for non-singular tool path planning by directly modifying the C -axis commands in the joint space using linear interpolation technique. Finally, both simulation and experimental results on a prototype machine show that in the neighborhood of singularity, continuous and smooth joint motions and acceptable tracking accuracy can be ensured after the modification.

Kinematics Analysis and Optimization of a 3-DOF Planar Tensegrity Manipulator under Workspace Constraint

Tensegrity mechanisms (TM) are well-appraised for their compliance and lightweight, making their design theory a hot research topic recently. However, due to unconstrained DOFs, the design and analysis of TMs are generally more complicated than traditional mechanisms composed of rigid links and joints. A compact 3-DOF tensegrity manipulator is introduced and an advanced two-step kinematic modeling method is proposed in this paper. This method is first assumed that bars and cables are rigid to estimate the equilibrium state using the energy-based method. Then, the flexibility of bars and cables is considered, and the force density method equations are solved utilizing the equilibrium state obtained by the previous step as the initial guess of iteration for fast computation. Based on the two-step method, the performances of the TM, such as workspace, manipulability, potential energy, and stiffness, are analyzed. Thereafter, the installation position and size of the manipulator are optimized under the workspace constraint. In the optimization process, discrete points on the prescribed task workspace contour are used to simplify the constraint to improve computational efficiency. Finally, study cases are investigated to validate the proposed method, and the feasibility of the discrete sampling method for constraint simplification is also verified.

Dynamic modeling and design of a 5-DOF hybrid robot for machining

This paper introduces a dynamic modeling and design approach for a novel 5-DOF (degrees of freedom) hybrid robot for machining. It mainly considers three performance indices: the workspace/machine volume ratio, the lateral stiffness, and lower modes, significantly affecting static and dynamic compliances of the robot at the reference configuration. The influences of design variables on the performance indices are systematically investigated, leading to the formulation of the design problem and a comprehensive design strategy for the parallel mechanism within the robot. Then, the proposed method is applied to the dynamic design of the robot, resulting in the development of a full-size prototype machine. Both the numerical simulation and experimental tests illustrate that the designed hybrid robot offers satisfactory static and dynamic behaviors within a relatively large task workspace, thereby fulfilling the essential requirements for robot machining.

Kinematic design of a translational parallel mechanism based on sub-kinematic chain determined workspace superposition

This paper presents the kinematic design of a translational parallel mechanism (PM) named Vari-Orthoglide by means of the workspace superposition, according to the sub-kinematic chain (SKC) based PM composition principle. The main topological characteristics of the manipulator with two SKCs under study, such as the position and orientation (POC) characteristics, degree of freedom (DOF) and coupling degree are analyzed, which turns out that the coupling degree equals to 1, implying the partially decoupled motion. With the topological characteristics based kinematic modeling principle, a symbolic model of the kinematics is established to derive its symbolic direct and inverse kinematic solutions. Based upon the direct kinematic solution, the workspaces for the two SKCs can be efficiently found. Moreover, the singularity loci are identified for finding the singularity-free workspace, where a regular workspace is fitted as the task workspace as expected. The presented work shows an approach to design translational parallel mechanisms considering motion decoupling and regular workspace, applicable to other types of parallel mechanisms.

Hierarchical design, laboratory prototype fabrication and machining tests of a novel 5-axis hybrid serial-parallel kinematic machine tool

Abstract Hybrid serial-parallel kinematic machine tool (HSPKMT) has been regarded as a promising solution for 5-axis machining in many industrial fields. A typical HSPKMT can be constructed by integrating a parallel functional module with a serial functional module. Following this way, the authors construct a novel 5-axis HSPKMT through the integration of an over constrained redundantly actuated parallel module with a stack-up serial gantry. The proposed HSPKMT can accomplish a 5-axis motion capacity with three translations and two rotations (3T2R). A hierarchical design method is proposed to facilitate the design issues of the 5-axis HSPKMT. According to the hierarchical method, a laboratory prototype is designed with a top-down strategy and then fabricated with a bottom-up strategy. An open-architecture computer numerical control (CNC) system is developed to drive the fabricated prototype. A kinematic analysis is carried out to reveal necessary kinematic properties of the proposed HSPKMT. The reachable workspace and task workspace are defined to graphically illustrate the machine's position-orientation capabilities. A workspace performance index is formulated to compare the proposed 5-axis HSPKMT with several 5-axis machine tools. Based the kinematic analysis, a 5-axis machining methodology is developed and further applied to the laboratory prototype to perform 5-axis machining tasks. The machining tests verify that the proposed novel HSPKMT possesses desirable 5-axis machining capability with the tolerance rang of ±0.05 mm. This also implies that the proposed hierarchical design method as well as the 5-axis machining methodology can be further applied to other types of HSPKMTs with minor modifications.

Human-in-the-Loop Control Strategy of Unilateral Exoskeleton Robots for Gait Rehabilitation

In this article, a human-in-the-loop control methodology is proposed for the gait rehabilitation of patients with hemiplegia. It utilizes a unilateral exoskeleton system consisting of a unilateral lower limb exoskeleton and a real-time robot follower, such that the affected legs can be coordinated with the healthy legs with the assistance of the exoskeleton robot. In order to achieve immersive training during the physical therapy, the human-in-the-loop controller is developed. Furthermore, a region-based barrier Lyapunov function (BLF) is designed to separate the task workspace of the exoskeleton into a human region and a robot region, enabling the human leg to follow the desired motion trajectory in a compliant region, and the motion control of the exoskeleton is determined by humans; while in the robot region, the exoskeleton dominates the movement of human subjects. In order to make the motion control transit smoothly between the robot region and the human region, an adaptive controller is exploited to counteract the system’s nonlinear uncertainties. Both the theoretical analysis and experimental results support the effectiveness and practicability on hemiplegic patients of our control strategy.

Optimal Robot Motion Planning of Redundant Robots in Machining and Additive Manufacturing Applications

The paper deals with the generation of optimal trajectories for industrial robots in machining and additive manufacturing applications. The proposed method uses an Ant Colony algorithm to solve a kinodynamic motion planning problem. It exploits the kinematic redundancy that is often present in these applications to optimize the execution of trajectory. At the same time, the robot kinematics and dynamics constraints are respected and robot collisions are avoided. To reduce the computational burden, the task workspace is discretized enabling the use of efficient network solver based on Ant Colony theory. The proposed method is validated in robotic milling and additive manufacturing real-world scenarios.

Co-Actuation: A Method for Achieving High Stiffness and Low Inertia for Haptic Devices.

Achieving high stiffness and low inertia is a big challenge for current haptic devices. Impedance-based devices are limited in providing high stiffness while, in contrast, admittance-based devices are limited in generating low inertia. Thus, it is difficult to simulate hard contact and small inertia simultaneously in virtual environments. In this paper, we introduce a co-actuation module to overcome this difficulty. The module is a one degree-of-freedom (DOF) revolute joint which consists of a link and a physical constraint with a clearance between the two components. A motor controls the physical constraint moving cooperatively with the link. In free space, the constraint has no contact to the link and thus, users can move the link freely without feeling the inertia of the motor. In constrained space, the constraint comes into contact with the link and thus, users can feel a resistance from the motor. By means of a direct physical contact between the link and the constraint, users can feel a hard virtual surface. This paper describes the principle and the implementation of the proposed co-actuation module. Performance evaluation was conducted using a two-DOF haptic device in a task workspace of 100 mm × 100 mm. The effective inertia of the device is 64-142 g within the task workspace. The device can stably render a virtual wall with stiffness as high as 65 N/mm. The penetration to the virtual wall was 0.02-0.41 mm when tapping the wall with a speed range of 80-320 mm/s. The maximum back driving force was about 0.19 N when moving within 4.5-8.6 mm/s. The experimental results demonstrate that the concept of co-actuation is feasible in achieving high force, high stiffness range and low inertia for haptic devices.

Human-Cooperative Control of a Wearable Walking Exoskeleton for Enhancing Climbing Stair Activities

In this paper, a human-cooperative control strategy is proposed for locomotion assistance along a physiologically meaningful path through a wearable walking exoskeleton. First, a kinematics model for climbing stairs with the step length, walking speed, and direction is developed. Then, the trajectory of the center of gravity (CoG) in both sagittal plane (SP) and coronal plane (CP) of the human–robot system is designed using virtual slope method (VSM) and dual length linear inverted pendulum model (DLLIPM), which are generated in parallel to the gradient vector of the virtual slope and the horizontal plane, respectively. Then, the task workspace of climbing stairs can be divided into a human domination region and a robot assistance region through a novel designed barrier energy function, such that the motion of human's legs can be constrained within a compliant region around the desired trajectories. Moreover, based on a smooth transition between the human and robot regions, an adaptive controller is designed to simultaneously incorporate robot motion and human's capabilities. Actual experiments involving several human subjects have been conducted and its results demonstrate the performance of the control strategy.

Conceptual design and dimensional synthesis of a novel parallel mechanism for lower-limb rehabilitation

SUMMARYThis paper introduces a novel 2R1T parallel manipulator redundantly actuated by pneumatic muscles for lower-limb rehabilitation. First, the conceptual design of the proposed 3-DOF parallel mechanism is presented. Then, the inverse kinematics and the generalized Jacobian analysis are carried out. Based on the generalized Jacobian and the constraint characteristics of the mechanism, the force/motion transmissibility of the redundantly actuated parallel mechanism is investigatedviafour individual cases without actuation redundancy, leading to a suitable local transmission index for the evaluation of kinematic performance of the proposed mechanism. Finally, the design variables are optimized by maximizing the mean value of the local transmission index with the aid of genetic algorithm. The numerical result shows that the proposed parallel mechanism can achieve a good kinematic performance in its task workspace.

Kinematic calibration of a 6-DOF hybrid robot by considering multicollinearity in the identification Jacobian

This paper presents a systematic approach for the kinematic calibration of a 6-DOF hybrid polishing robot. It concentrates particularly on dealing with ill-conditioning that arises from multicollinearity in the identification Jacobian as a consequence of limited pose measurements. A linearized error model is formulated, using screw theory, by considering all possible source errors at the joint/link level. Calibration is based upon pose error measurements captured using a laser tracker when the polishing head undergoes pure translation within the task workspace. A two-step procedure for error parameter estimation and pose error compensation is then proposed: (1) coarse estimation and compensation of the encoder offsets using linear least squares; and (2) fine estimation of the whole set of identifiable error parameters using a Liu estimation and subsequent modification of the NC trajectory dataset of the polishing head. Both simulations and experiments on a prototype machine show that the overall standard deviation of the error parameters identified by Liu estimator is much less than that estimated by linear least squares, confirming its greater robustness in the presence of measurement uncertainty. The proposed approach results in satisfactory pose accuracy of the polishing head throughout the entire task workspace.

An engineering-oriented motion accuracy fluctuation suppression method of a hybrid spray-painting robot considering dynamics

Due to the complex dynamic characteristics of a hybrid spray-painting robot, the motion accuracy fluctuates in the workspace. In order to reduce the accuracy fluctuation of the hybrid robot, this paper proposes an engineering-oriented motion accuracy assurance method by considering dynamic characteristics, including workspace optimization and control parameters design. First, the dynamic model of the hybrid spray-painting robot is derived based on the virtual work principle, and a dynamic evaluation index is investigated to describe the possible maximum dynamic load. Then, based on the evaluation index, the relative location of the task workspace for painting an aircraft wing in the whole workspace is optimized. Then, a control parameters design approach is presented to ensure that the robot always has desired accuracy in the optimized task workspace. Finally, some experiments are performed, and the results show that the robot has better performance in the optimized task workspace with the designed control parameters, which proves the effectiveness of the proposed method.

Kinematic Calibration Based on the Multicollinearity Diagnosis of a 6-DOF Polishing Hybrid Robot Using a Laser Tracker

A general methodology for ensuring the geometric accuracy of a 6-DOF polishing hybrid robot having a 3UPS and UP parallel mechanism and a 3-DOF wrist is presented. The process is implemented in three steps: formulation of the error model containing complete source errors such as offset errors of the actuated joints and structural errors of the joints and links utilizing product of exponentials formula and screw theory. Measurement of the full pose error twist with a specially designed measurement tool having three reference points was undertaken. Identification of the source errors by a stepwise identification strategy to overcome the ill-conditioned problem arising from the multicollinearity and development of a linearized error compensator was completed. An experiment has been carried out on the prototype, and the results show that, after calibration, a position volumetric error of 0.07 mm and an orientation error of 0.07 degrees can be achieved over the cubic task workspace with repeatability of 0.016 mm and 0.010 degrees.

Workspace Analysis of a Hybrid Kinematic Machine Tool with High Rotational Applications

This paper presents a novel parallel manipulator with one translational and two rotational (1T2R) degrees of freedom that can be employed to form a five-degree-of-freedom hybrid kinematic machine tool for large heterogeneous complex structural component machining in aerospace field. Compared with serial or parallel machine, hybrid machine has the merits of high stiffness, high speed, large workspace, and complicated surface processing ability. To increase stiffness, three-degree-of-freedom redundantly actuated and overconstrained 2PRU-PRPS parallel manipulator (P denotes the active prismatic joint) is proposed, which is utilized as the main body of hybrid machine. By resorting to the screw theory, the degree of freedom of the proposed mechanism is briefly addressed including the initial configuration and general configuration and validated by Grübler-Kutzbach (G-K) equation. Next, kinematic inverse solution and parasitic motion of the parallel manipulator are deduced and the transformational relations between the Euler angle and Tilt-Torsion (T-T) angle are identified. Thirdly, the performance evaluation index of orientation workspace is introduced, and the reachable workspace and joint workspace are formulated. Through specific examples, the reachable workspace, task workspace, and joint workspace of the redundant actuation parallel manipulator are depicted. Compared with overstrained 2PRU-PRS parallel manipulator, corresponding analyses illustrate that the proposed parallel manipulator owns much better orientation capability and is very meaningful to the development of the five-axis hybrid machine tool.

Stiffness modeling and analysis of a novel 5-DOF hybrid robot

This paper deals with the stiffness modeling and analysis of a novel 5-DOF hybrid robot named TriMule which is composed of a 3-DOF positioning parallel mechanism plus a 2-DOF wrist. The robot is especially designed as a compact yet rigid module suitable for large part on-site machining. Mainly drawing on screw theory, a semi-analytic stiffness model of the robot is formulated by taking into account the component compliances associated with the elements of both the parallel mechanism and the wrist, resulting in the Cartesian stiffness matrix that can explicitly be expressed in terms of the compliance matrices down to the joint and link level. The stiffness distributions of the tool head over a prescribed task workspace are predicted and the contributions of joint/link compliances are evaluated using a set of global indices.