INSTANTANEOUS KINEMATIC CHARACTERISTICS OF ASPECIAL 3-UPU PARALLEL MANIPULATOR

The instantaneous kinematics of a special 3-UPU parallel platform manipulator is discussed. First, the instantaneous motions of the 3-UPU manipulator in four kinds of positions and a special manipulator are studied by reciprocal screw theory. Then, the principal screws in one of four positions are obtained. It is shown that the moving platform has five degrees of freedom （DOF） in the initial position or after a translation along the z-axis; In the generic position, the mechanism only has three DOF, moreover the three DOF characteristics are different in different position. The instantaneous kinematic characteristics of alike 3-UPU mechanisms are very different in different position and special structure. The results presented are important to the use of alike 3-UPU parallel manipulator and contribute to the mechanism theory .

Inverse kinematics of a heavy duty manipulator with 6-DOF based on dual quaternion

An iterative method is introduced successfully to solve the inverse kinematics of a 6-DOF manipulator of a tunnel drilling rig based on dual quaternion, which is difficult to get the solution by Denavit-Hartenberg(D-H) based methods. By the intuitive expression of dual quaternion to the orientation of rigid body, the coordinate frames assigned to each joint are established all in the same orientation, which does not need to use the D-H procedure. The compact and simple form of kinematic equations, consisting of position equations and orientation equations, is also the consequence of dual quaternion calculations. The iterative process is basically of two steps which are related to solving the position equations and orientation equations correspondingly. First, assume an initial value of the iterative variable; then, the position equations can be solved because of the reduced number of unknown variables in the position equations and the orientation equations can be solved by applying the solution from the position equations, which obtains an updated value for the iterative variable; finally, repeat the procedure by using the updated iterative variable to the position equations till the prescribed accuracy is obtained. The method proposed has a clear geometric meaning, and the algorithm is simple and direct. Simulation for 100 poses of the end frame shows that the average running time of inverse kinematics calculation for each demanded pose of end-effector is 7.2 ms on an ordinary laptop, which is good enough for practical use. The iteration counts 2-4 cycles generally, which is a quick convergence. The method proposed here has been successfully used in the project of automating a hydraulic rig.

A novel symmetrical 3-DOF PKM and its performance comparison with 3-PRS PKM

A novel symmetrical 3-degree-of-freedom(DOF) parallel kinematic manipulator(PKM) is firstly presented,which is named 3-P(Qu) RU.According to the structure feature,a double closed loop vector method is proposed to investigate this PKM.Based on this method,kinematic,velocity and error models of this manipulator are established respectively.Since3-PRS PKM has been applied successfully in practice and its structure is similar to the 3-P(Qu) RU PKM,corresponding models of a 3-PRS PKM are given and a performance comparison study between them is investigated on workspace,manipulator dexterity,position error and error sensitivity.The comparison results reveal that the 3-P(Qu) RU PKM has the advantage on velocity performance and the disadvantage on accuracy performance.This novel 3-P(Qu) RU PKM is an available selection for a tool head of a hybrid machine tool and the analysis is greatly helpful for the further applications of this manipulator.

Optimum calibration of a parallel kinematic manipulator using digital indicators

This paper presents a calibration method for parallel manipulators using a measurement system specially installed on an external fixed frame. The external fixed frame is important as an error reference for calibration in certain operations, such as in the configuration of a parallel manip- ulator functioning as a machine tool where the workpiece is fixed to a worktable. The pose of the end-effector is mea- sured using three digital indicators installed on the external fixed frame. To enable measurement, the end-effector is assumed to be a plane large enough that all digital indicators could touch. The error is defined as the difference between the theoretical and actual readings of the digital indicators. The geometric parameters of the parallel manipulator are optimized to minimize this error. This calibration method is low cost and feasible for compensating geometric parameter errors for a parallel manipulator. Optimal pose selection for the calibration is achieved using a swarm intelligence search algorithm. The method is implemented on a prototype of a six degrees-of-freedom （DOFs） Gough-Stewart platform constructed to function as a machine tool.

Deflection estimation of industrial robots with flexible joints

The application of industrial robots in manufacturing industries has received considerable concerns due to the high flexibility,multifunctionality,and cost-efficiency.It is well known that the robot positioning accuracy is susceptible to the load and motion of robots owing to the insufficient stiffness of robots.Therefore,the machining accuracy improvement has been a research focus in the robotic manufacturing industries in the last decade.To overcome the measurement difficulty of the joint torque and position as well as the complex dynamic coupling between rotors and links,two forward dynamics algorithms for the robot deflection estimation are proposed in this paper.The robot kinematics and dynamics algorithms considering the dynamic coupling between rotors and links are developed based on Lie theory.The forward dynamics equations of robots are solved via the proposed algorithms:the implicit numerical integration algorithm and numerical iterative estimation algorithm.When only the motor position is available,the implicit numerical integration algorithm is employed to solve the forward dynamics equations to estimate the joint torque and position.At the same time,when both the motor position and torque are available,the forward dynamics equations can be reorganized as algebraic equations and solved by the numerical iterative estimation algorithm.Simulations of a 6-DOF serial robot are performed to verify the accuracy of the proposed algorithms.