Hongzhou Jiang

A Survey on Control of Parallel Manipulator

Based on extensive study on literatures of control of parallel manipulators and serial manipulators, control strategies such as computed torque control, PD+ control, PD with feedforward compensation, nonlinear adaptive control are classified into two categories: model-based control and performance-based control. Besides, as advanced control strategies, robust control and passivity-based control for the parallel manipulators are also introduced. Comparative study in view of computation burden and tracking performance are performed. It turned out that the physical structure properties of parallel manipulators’ dynamics are similar with that of serial ones, and this constitutes a common foundation for the two kinds of manipulators to develop together that control design of parallel manipulators can start with ever established control methods of serial manipulators.

Optimal design of a class of generalized symmetric gough-stewart parallel manipulators with dynamic isotropy and singularity-free workspace

In this paper, the definition of generalized symmetric Gough-Stewart parallel manipulators is presented. The concept of dynamic isotropy is proposed and the singular values of the bandwidth matrix are introduced to evaluate dynamic isotropy and solved analytically. Considering the payloads mass-geometry characteristics, the formulations for completely dynamic isotropy are derived in close form. It is proven that a generalized symmetric Gough-Stewart parallel manipulator is easer to achieve dynamic isotropy and applicable in engineering applications. An optimization procedure based on particle swarm optimization is proposed to obtain better dexterity and large singularity-free workspace, which guarantees the optimal solution and gives mechanically feasible realization.

Study on dynamic isotropy of a class of symmetric spatial parallel mechanisms with actuation redundancy

In this paper, we study dynamic isotropy using natural frequency analysis for a class of symmetric spatial parallel mechanisms (SSPMs) with 2p (p≥3) struts. This kind of dynamic isotropy has been defined as the square roots of the eigenvalues of the equivalent mass-spring systems formed via the interactions between rigid-body mechanical systems and their driving systems. Analytic expressions for these eigenvalues are then derived in the task space, which is linearly dependent on p. Furthermore, a general compliance center was found for all the SSPMs in which the parallel mechanisms are fully decoupled. Based on the dynamically decoupling, then, dynamic isotropy for the SSPM is discussed which shows that though taking the inertial parameters into consideration, the SSPM can not attain complete dynamic isotropy and the optimal dynamic isotropy index is the quartic root of two. At the end of the paper, to demonstrate these results, an example is given.

Model-based Control for 6-DOF Parallel Manipulator

A novel model-based controller for six-degree-of- freedom (DOF) parallel manipulator is proposed in this paper, in order to abatement the influence of platform load variety and compel the steady state errors converge to zero. In this paper, 6-DOF parallel manipulator is described as multi-rigid-body systems, the mathematical model of the 6-DOF parallel manipulator including dynamics based on Kane method and kinematics used closed-form solutions and Newton-Raphson method is built in generalized coordinate system. The model-based controller is presented with the feedback of cylinders positions of platform, desired trajectories and dynamics gravity as the input and the servovalve current as its output. The performance of the control scheme for 6-DOF parallel manipulator is analyzed. Simulation of the parallel manipulator with model-based controller is executed in MATLAB/Simulink, the simulation results indicate the model-based controller can reduce the influence of load variety of platform and eliminate steady state error of 6-DOF hydraulic driven parallel manipulator.

Modal control of a hydraulically driven redundant actuated fully parallel mechanism

In this paper, we consider the problem of modal-space control for the hydraulically driven fully parallel mechanism with actuation redundancy. Firstly, the mechanical-hydraulic interaction system is transformed into modal-space model. Then, independent modal decoupled systems for the redundant mechanism are obtained. According to the eigenvalue frequency characteristics, two types of modal systems – zero eigenvalue modal systems and nonzero ones – naturally result and these systems can be treated separately. For the nonzero eigenvalue modal systems, it is convenient to employ dynamic pressure feedback control to regulate damping. For the zero eigenvalue modal systems, we give a proof to show that they just lie in the null space of the Jacobian of the mechanism, then it is more appropriate to implement force control for this type of modal system. Some simulation results further explain the proposed modal-space control method.

ANN-based Acceleration Harmonic Identification for an Electro-hydraulic Servo System

Since the dead zone phenomenon occurs in electro-hydraulic servo system, the acceleration output of the system corresponding to a sinusoidal input contains higher harmonic besides the fundamental response, causing harmonic distortion of the output acceleration signal. The output wave includes odd harmonics up to 11th harmonic. The method for harmonic identification based on artificial neural network (ANN) is proposed here. This method uses an Adaline neural network to identify the amplitude and phase of harmonics as well as the fundamental acceleration output on-line. The weights of the Adaline are adjusted according to the error between the actual and the estimated acceleration to yield the Fourier coefficients of the output wave. The simulation results show the validity of the analytical results and the ability of the algorithm to on-line identify all harmonics including the fundamental effectively with high accuracy.

Precision Gravity Center Position Measurement System for Heavy Vehicles

Knowledge of a vehicle’s inertial parameters is essential for safety research and accident reconstruction. A precision measure system is proposed to determine the weight and gravity center for heavy vehicles. Based on a static gravity measuring principle with three measuring points, a hydraulically driven 2-DOF motion platform is developed. The transfer function model is derived for the hydraulically driven system. By means of a degree-of-freedom control scheme, the platform can realize accurate positioning to construct two intersected planes and work out the three-dimensional coordinates of the vehicle gravity center. Experiments demonstrate that the system has less than 0.3% measurement error in weight, and is able to measure the gravity centre accurately with deviation ≤3mm in X and Y direction, and ≤5mm in Z direction.

Optimal Design of an Orthogonal Generalized Parallel Manipulator Based on Swarm Particle Optimization Algorithm

Study of methods to design optimal Gough–Stewart parallel manipulator geometries meeting orthogonality is of high interest. The paper will design the Orthogonal Generalized Gough-Stewart Parallel Manipulator (OGGSPM) based on the composite hyperboloids. Moreover, Particle Swarm Optimization (PSO) algorithm is introduced to perform the structure optimization as the Jacobian matrix condition number is used as the evaluation index. As an example, two different optimization cases are presented.

Analysis for Rotation Orthogonality of a Dynamically Adjusting Generalized Gough-Stewart Parallel Manipulator

A Gough-Stewart parallel manipulator using point decoupled design method is orthogonal only at a single point, which means that it has small high-precision workspace. It is hard to break through its’ restriction to be applied in the field of engineer applications with larger workspace. This paper formulates the problem to guarantee continuous orthogonality when the manipulator rotating along the z-axis, called rotation orthogonality. Compared to traditional optimum, the rotation orthogonality leads to better performances with enlarged workspace. A class of dynamically adjusting generalized Gough-Stewart parallel manipulators (DAGGSPM) is proposed. The dynamically adjusting mechanism for rotation orthogonality and the related algorithm are deduced analytically. Through analysis of three typical cases and numerical verifications, the results show that a DAGGSPM can pave the way for high-precision applications with large scale workspace including laser weapon pointing, scanning microscopes and integrated circuit fabrication.

Design of a Maximally Regular Acceleration Sensor Based on Generalized Gough-Stewart Platforms

An analytical formulation and a new routine are presented to design a maximally regular acceleration sensor based on a generalized Gough-Stewart platform. The Jacobian matrix considering the measuring point is constructed symbolically and also acceleration mapping matrix done. Optimal indices and coupling evaluation are proposed to qualify performances of the sensor. Subsequently, the singular values of the acceleration Jacobian matrix are solved analytically to evaluate acceleration transmission. The conditions for the maximally regular acceleration are expressed in close-form. Based on the analytical formulation, an optimal design routine is put forward to determine a family of orthogonal and maximally regular acceleration sensor. With the aid of numerical examples, the acceleration-coupling is investigated and the results illustrate that the proposed design method is effective.