Xuechao Duan

Calibration and Motion Control of a Cable-Driven Parallel Manipulator Based Triple-Level Spatial Positioner

This paper deals with the kinematic calibration and motion control of a triple-level spatial positioner consisting of the cable-driven parallel manipulator (CDPM), active gyro stabilizer (AGS), and the Stewart platform. A six-degree-of-freedom laser tracker is employed when calibrating the benchmark positions and measuring the real-time position and orientation in motion control, which makes it a straightforward solution to tackle with hierarchical mechatronic system actuated by servomotors with incremental encoders. Then the trajectory planning and motion control of the triple-level robotic spatial positioner are explored to verify the correctness and to what extent the calibration improves the system. This CDPM based spatial positioner has an accuracy of several millimeters though it has a ten-meter workspace.

Motion prediction and supervisory control of the macro-micro parallel manipulator system

This paper deals with the motion prediction and control of the macro-micro parallel manipulator system for a 500-m-aperture spherical radio telescope (FAST). Firstly, based on principles of parallel mechanism, a decoupled tracking and prediction algorithm to predict the position and orientation of the movable macro parallel manipulator is presented in this paper. Then, taken as the upper layer supervisory controller in the joint space of the micro parallel manipulator, the adaptive interaction PID controller utilizing the adaptive interaction algorithm to adjust the parameters of a canonical PID controller is discussed. In addition, the digital servo filters with feedforward are employed in the linear actuators as the lower layer controllers. Experimental results of a one-tenth scale FAST field model validate the effectiveness of the supervisory controller and the motion prediction algorithm.

Optimization of the workspace of a MEMS hexapod nanopositioner using an adaptive genetic algorithm

This paper presents workspace optimization of a MEMS flexure-based hexapod nanopositioner previously built by the National Institute of Standards and Technology (NIST). Workspace is one of the most important quality criteria for positioning devices. Given a lot of literature on workspace optimization of rigid body parallel robots, there is relatively less work done in their compliant counterparts due to the challenges in determining the workspace. In this paper, we present an analytical formulation and a search algorithm to determine the workspace of the flexure based parallel mechanisms. A novel adaptive genetic algorithm has been developed to conduct the single and bi-objective optimization for maximum translational and rotational workspace. These optimization results provide a guidance for the designer to improve the device for specific design requirements.

Dynamic Analysis and Vibration Attenuation of Cable-Driven Parallel Manipulators for Large Workspace Applications

Cable-driven parallel manipulators are one of the best solutions to achieving large workspace since flexible cables can be easily stored on reels. However, due to the negligible flexural stiffness of cables, long cables will unavoidably vibrate during operation for large workspace applications. In this paper a finite element model for cable-driven parallel manipulators is proposed to mimic small amplitude vibration of cables around their desired position. Output feedback of the cable tension variation at the end of the end-effector is utilized to design the vibration attenuation controller which aims at attenuating the vibration of cables by slightly varying the cable length, thus decreasing its effect on the end-effector. When cable vibration is attenuated, motion controller could be designed for implementing precise large motion to track given trajectories. A numerical example is presented to demonstrate the dynamic model and the control algorithm.

Workspace Classification and Quantification Calculations of Cable-Driven Parallel Robots:

Large workspace is one of the promising advantages possessed by the cable-driven parallel robots (CDPR) over the conventional rigid-link robots. This paper focuses on the dynamic analysis and workspace classification based on the general motion equation of cable robot and the unilateral property of cables. The combinations of different types of two conditions lead to several different types of workspace, including static equilibrium workspace, wrench closure workspace, wrench feasible workspace, dynamic workspace, and collision-free workspace. A qualitative comparison of different types of workspaces is performed. The simulation results verify the relationship between the several types of workspaces.

The Application Research of Inverse Finite Element Method for Frame Deformation Estimation

A frame deformation estimation algorithm is investigated for the purpose of real-time control and health monitoring of flexible lightweight aerospace structures. The inverse finite element method (iFEM) for beam deformation estimation was recently proposed by Gherlone and his collaborators. The methodology uses a least squares principle involving section strains of Timoshenko theory for stretching, torsion, bending, and transverse shearing. The proposed methodology is based on stain-displacement relations only, without invoking force equilibrium. Thus, the displacement fields can be reconstructed without the knowledge of structural mode shapes, material properties, and applied loading. In this paper, the number of the locations where the section strains are evaluated in the iFEM is discussed firstly, and the algorithm is subsequently investigated through a simple supplied beam and an experimental aluminum wing-like frame model in the loading case of end-node force. The estimation results from the iFEM are compared with reference displacements from optical measurement and computational analysis, and the accuracy of the algorithm estimation is quantified by the root-mean-square error and percentage difference error.

Kinematic Analysis of a Hybrid Structure

This paper presents a kinematic analysis and simulation of a hybrid structure applied to the new design cable-suspended feed structure (CSFS) for the next generation of large spherical radio telescopes. First, considering the requirement that feeds should be tilted from 40° to 60° and that the tracking precision in steady state is 4mm, a novel design of the feed supporting structure including a cable-cabin structure, an AB axis structure and a Stewart platform is performed. Next, kinematic analysis and the simulation of the CSFS are done. Simulations have been developed in combination with the 50m CSFS model, which demonstrate the effectiveness and feasibility of the proposed three-level cable-suspended feed system.

Short-Term Power Load Forecasting Method Based on Improved Exponential Smoothing Grey Model

In order to improve the prediction accuracy, this paper proposes a short-term power load forecasting method based on the improved exponential smoothing grey model. It firstly determines the main factor affecting the power load using the grey correlation analysis. It then conducts power load forecasting using the improved multivariable grey model. The improved prediction model firstly carries out the smoothing processing of the original power load data using the first exponential smoothing method. Secondly, the grey prediction model with an optimized background value is established using the smoothed sequence which agrees with the exponential trend. Finally, the inverse exponential smoothing method is employed to restore the predicted value. The first exponential smoothing model uses the 0.618 method to search for the optimal smooth coefficient. The prediction model can take the effects of the influencing factors on the power load into consideration. The simulated results show that the proposed prediction algorithm has a satisfactory prediction effect and meets the requirements of short-term power load forecasting. This research not only further improves the accuracy and reliability of short-term power load forecasting but also extends the application scope of the grey prediction model and shortens the search interval.

Modeling and Analysis of a 2-DOF Spherical Parallel Manipulator

The kinematics of a two rotational degrees-of-freedom (DOF) spherical parallel manipulator (SPM) is developed based on the coordinate transformation approach and the cosine rule of a trihedral angle. The angular displacement, angular velocity, and angular acceleration between the actuators and end-effector are thus determined. Moreover, the dynamic model of the 2-DOF SPM is established by using the virtual work principle and the first-order influence coefficient matrix of the manipulator. Eventually, a typical motion plan and simulations are carried out, and the actuating torque needed for these motions are worked out by employing the derived inverse dynamic equations. In addition, an analysis of the mechanical characteristics of the parallel manipulator is made. This study lays a solid base for the control of the 2-DOF SPM, and also provides the possibility of using this kind of spherical manipulator as a 2-DOF orientation, angular velocity, or even torque sensor.

Complete real solution of the five-orientation motion generation problem for a spherical four-bar linkage

For a spherical four-bar linkage, the maximum number of the spherical RR dyad (R: revolute joint) of five-orientation motion generation can be at most 6. However, complete real solution of this problem has seldom been studied. In order to obtain six real RR dyads, based on Strum’s theorem, the relationships between the design parameters are derived from a 6th-degree univariate polynomial equation that is deduced from the constraint equations of the spherical RR dyad by using Dixon resultant method. Moreover, the Grashof condition and the circuit defect condition are taken into account. Given the relationships between the design parameters and the aforementioned two conditions, two objective functions are constructed and optimized by the adaptive genetic algorithm(AGA). Two examples with six real spherical RR dyads are obtained by optimization, and the results verify the feasibility of the proposed method. The paper provides a method to synthesize the complete real solution of the five-orientation motion generation, which is also applicable to the problem that deduces to a univariate polynomial equation and requires the generation of as many as real roots.