Zhufeng Shao

A Fuzzy PID Approach for the Vibration Control of the FSPM

This paper focuses on the vibration control issue of a Flexibly Supported Parallel Manipulator (FSPM), which consists of a flexible support and a rigid parallel manipulator. The distinct characteristic of an FSPM is the dynamic coupling between the rigid and flexible parts, which challenges the vibration control implemented by the rigid parallel manipulator. The research object is a 40m scale model of the Feed Support System (FSS) for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) project, which is composed of a cable-driven parallel manipulator, an A-B rotator and a rigid Stewart manipulator, assembled in series. The cable-driven parallel manipulator is sensitive to disturbances and could lead to system vibration with a large terminal error. The rigid Stewart manipulator is designed to carry out the vibration control. Considering the time-variability, nonlinearity and dynamic coupling of an FSPM, a fuzzy proportional–integral–derivative (PID) controller is introduced. The fuzzy inference rules established on the terminal error and the error change are used to adjust the PID parameters to achieve better performance. Physical experiments are carried out and the results indicate that the fuzzy PID method can effectively promote the terminal precision and maintain system stability. The control methodology proposed in this paper is quite promising for the vibration control of an FSPM.

Driving force analysis for the secondary adjustable system in fast

The Secondary Adjustable System (SAS) addressed here is a central component of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). It is a 6-degree-of-freedom rigid Stewart manipulator, in which one platform (the end-effector) should be controlled to track-desired trajectory when another platform (denoted as the base) is moving. Driving force analysis of the SAS is the basis for selecting rational servomotors and guaranteeing the dynamic performance, which will affect the terminal pose accuracy of the FAST. In order to determine the driving forces of the SAS, using the Newton-Euler method, the inverse dynamics of the Stewart manipulator is modeled by considering the motion of the base. Compared with the traditional dynamic models, the inverse dynamic model introduced here possesses an inherent wider application range. By adopting the kinematic and dynamic parameters of the FAST prototype, the driving force analysis of the SAS is carried out, and the driving force optimization strategies are proposed. Calculation and analysis presented in the paper reveal that there are three main factors affecting the driving forces of the SAS. In addition, the driving force analysis of this paper lays out guidelines for the design and control of the FAST prototype, as well as the structure and trajectory optimization.

Atlas based kinematic optimum design of the Stewart parallel manipulator

Optimum design is a key approach to make full use of potential advantages of a parallel manipulator. The optimum design of multi-parameter parallel manipulators(more than three design parameters), such as Stewart manipulator, relies on analysis based and algorithm based optimum design methods, which fall to be accurate or intuitive. To solve this problem and achieve both accurate and intuition, atlas based optimum design of a general Stewart parallel manipulator is established, with rational selection of design parameters. Based on the defined spherical usable workspace(SUW), primary kinematic performance indices of the Stewart manipulator, involving workspace and condition number are introduced and analyzed. Then, corresponding performance atlases are drawn with the established non-dimensional design space, and impact of joint distribution angles on the manipulator performance is analyzed and illustrated. At last, an example on atlas based optimum design of the Stewart manipulator is accomplished to illustrate the optimum design process, considering the end-effector posture. Deduced atlases can be flexibly applied to both quantitative and qualitative analysis to get the desired optimal design for the Stewart manipulator with respect to related performance requirements. Besides, the established optimum design method can be further applied to other multi-parameter parallel manipulators.

Optimal Design of a 3-DOF Cable-Driven Upper Arm Exoskeleton

With outstanding advantages, such as large workspace, flexibility, and lightweight and low inertia, cable-driven parallel manipulator shows great potential for application as the exoskeleton rehabilitation robot. However, the optimal design is still a challenging problem to be solved. In this paper, the optimal design of a 3-DOF (3-degree-of-freedom) cable-driven upper arm exoskeleton is accomplished considering the force exerted on the arm. After analysis of the working conditions, two promising configurations of the cable-driven upper arm exoskeleton are put forward and design parameters are simplified. Then, candidate ranges of two angle parameters are determined with the proposed main workspace requirement. Further, global force indices are defined to evaluate the force applied to the arm by the exoskeleton, in order to enhance the system safety and comfort. Finally, the optimal design of each configuration is obtained with proposed force indices. In addition, atlases and charts given in this paper well illustrate trends of workspace and force with different values of design parameters.

Design and Analysis of a Wire-Driven Parallel Mechanism for Low-Gravity Environment Simulation

Traditional simulation mechanisms are unable to meet the simulation requirements of spacecraft launcher in low-gravity environment, like in the Moon. Based on the advantages of wire-driven parallel mechanism, a 6-DOF low-gravity environment simulation device with eight wires is designed in this paper. Firstly, the configuration and dimensional design of this wire-driven parallel mechanism are carried out. To operate and control the wire-driven parallel mechanism, a force distribution algorithm is introduced and the workspace is analyzed. Then, an evaluation index named quality index is established in order to study the performance of the wire-driven parallel manipulator in its workspace and reasonable tension is obtained after analyzing the influence on quality index caused by different wire tension.

Optimum Design of 3-3 Stewart Platform Considering Inertia Property

Optimum design is a pivotal approach to fulfill the potential advantages of the parallel manipulator in practical applications. This paper concerns the optimum design issue of the 3-3 Stewart platform considering the inertia property, in addition to the kinematic performance. On the basis of spherical usable workspace, global conditioning index (GCI) is analyzed. Atlases of the workspace and GCI are deduced with the established nondimensional design space. Further, after dynamic modeling, the global inertia index (GII) is deduced from the joint-space inertia matrix, and corresponding GII atlases are drawn. In particular, an example is presented to illustrate the process of obtaining the practical optimum results based on these non-dimensional atlases. Since both kinematic and dynamic properties are considered, the optimum result will possess comprehensive performance improvements.

Dynamics Verification Experiment of the Stewart Parallel Manipulator

As the basis of dynamic analysis and driving force calculation, dynamic models and dynamic parameters are important issues in mechanical design and control. In this paper, a dynamics verification experiment, which covers both dynamic models and dynamic parameters as a whole, is carried out on the typical Stewart parallel manipulator. First, the complete dynamic model of the Stewart manipulator is derived, considering the force sensors. The Newton-Euler method with clear physical meaning is adopted to facilitate understanding and parameter definitions. The dynamic parameters are deduced based on the established three-dimensional virtual prototype and adjusted with actual measurements. The recorded trajectory, instead of the theory trajectory, is adopted to calculate the theoretical limb forces. The practical limb forces are measured using pull pressure sensors. Finally, the dynamic model and identified parameters are verified by comparing the limb forces obtained using the above two approaches. Experiment results show that theoretical and practical limb forces coincide well, with a small maximum RMS (root mean square) error of 1.516N and forces ranging from 10N to 40N. Additionally, the established dynamics verification algorithm, which involves dynamic modelling, a parameter identification approach and a data analysis method, are generic and practical, and can be flexibly applied to the dynamic analysis of other parallel manipulators.

Self-excited Vibration Analysis for the Feed Support System in FAST

China is currently building the largest single dish radio telescope in the world, which is called the Five-hundred meter Aperture Spherical radio Telescope (FAST). The feed support system in the FAST is composed of a cable-driven parallel manipulator, an A-B rotator, and a Stewart platform. Since the stiffness of the cable-driven parallel manipulator is low, the feed support system is prone to vibrate under the action of the Stewart platform. The main purpose of this paper is to study the self-excited vibration of the feed support system. Self-excited vibration involves the natural frequencies of the system and the resultant forces produced by the motion of the Stewart platform. This paper linearizes the dynamic equations of the system at an operating point and determines the configuration-dependent natural frequencies in the given workspace. This paper obtains the resultant forces from the legs due to the motion of the Stewart platform by using the numerical method. According to the natural frequencies and the resultant forces, the condition of self-excited vibration is given and verified by simulations. In order to verify the linearization method, an experimental platform of a cable-driven parallel manipulator is set up. The experimental results match well with the theoretical arithmetic. This paper provides a reference point for further studies on vibration suppression in the FAST.

The Structure and Dimensional Design of a Reconfigurable PKM

Parallel Kinematic Machines (PKMs) have many advantages and have been widely used in the machine industry. Benefitting from its modular structure, a PKM is more reconfigurable than traditional serial machines. In this paper, a new type of driving strut module and innovative joints are designed for the Reconfigurable Parallel Kinematic Machine (RPKM). The new driving strut module can be changed from linear drive mode to telescopic drive mode easily, and the new spherical joint and universal joint are designed to achieve a large rotation angle. The inverse kinematics problems in relation to the 6-DOF RPKM are analysed, and the Workspace Volume Index (WVI) and the Global ConditionIndex (GCI) are adopted to design the RPKM. According to the WVI and GCI analysis of the selected parameters for two types of 6-DOF PKM, the dimensional parameters of the RPKM are designed. In the end, the new type of RPKM prototype is built, with which a wax pattern is machined.

Research on Longitudinal Vibration Characteristic of the Six-Cable-Driven Parallel Manipulator in FAST

The first adjustable feed support system in FAST is a six-cable-driven parallel manipulator. Due to flexibility of the cables, the cable-driven parallel manipulator bears a concern of possible vibration caused by wind disturbance or internal force from the fine drive system. The purpose of this paper is to analyze vibration characteristic of the six-cable-driven parallel manipulator in FAST. The tension equilibrium equation of the six-cable-driven parallel manipulator is set up regarding the cables as catenaries. Then, vibration equation is established considering the longitudinal vibration of the cables. On this basis, the natural frequencies are depicted in figures since both analytical and numerical solutions are ineffective. Influence of the sags of the cables on the natural frequencies is discussed. It is shown that the sags of the cables will decrease the natural frequencies of the six-cable-driven parallel manipulator. Simplification to acquire the natural frequencies is proposed in this paper. The results justify effectiveness of the simplification to calculate the first-order natural frequencies. Distribution of the first-order natural frequencies in the required workspace is provided based on the simplification method. Finally, parameters optimization is implemented in terms of natural frequencies for building the six-cable-driven parallel manipulator in FAST.