Showing papers by "Zexiang Li published in 2006"

A Novel 3-DoF Purely Translational Parallel Mechanism

Abstract: A novel 3-DoF purely translational parallel mechanism, the Orthotripod, is proposed. It is a variant of the tripod based parallel machine and has a similar architecture to the Orthoglide. In order to reduce the number of passive joints and remove the effect of ease of abrasion of revolute joints, spherical joints are applied in the parallelogram. A mathematic mobility analysis shows the mechanism is indeed 3-DoF purely translational. We optimally design the Orthotripod and the tripod based parallel machine by maximizing the well-conditioned workspace. The optimized Orthotripod possesses a nearly ball-shaped workspace and has much better kinematic performance than the optimized tripod based parallel mechanism. The proposed mechanism is adaptable for machine tool applications

Task Space Based Contouring Control of Parallel Machining Systems

Abstract: Since the tracking error does not truly reflect product quality, the contouring error is introduced in the dynamic control of parallel machining systems. For real-time computation reason, the contouring error is approximated by the distance from the actual position to the tangent plane of the desired contour at the corresponding desired position, i.e., the error in normal direction. By attaching a moving task frame to each point on a desired trajectory, the tracking error is decomposed into tangential error and normal error. By the transformation introduced by the task frame, we obtain error dynamics in the task frame. The error dynamics is decoupled into error dynamics in tangential and normal directions by applying the computed torque control and choosing appropriate system matrices. Simulation shows that a larger bandwidth of the normal dynamics leads to smaller contouring error given fixed natural frequency for the tangential dynamics. By a comparison with the PD control in the world frame, the task space based contouring control exhibits much better performance in contouring accuracy

Geometric Contouring Control on the Smooth Surface

Abstract: In this paper we concentrate on the contouring control for the surface machining. The object of the motion control system is tracking the spatial curve lying on the surface. Observing that the contour error can be approximated by the tracking error (projected to the normal subspace of the surface), we propose a new design procedure based on the geometrical properties of the curves and surfaces. Essentially the controllers look ahead using the information provided by the curvature of the curves and surfaces. The simulation results show the efficiency of the design method.

Adaptive Contouring Control for High-Accuracy Tracking Systems

Abstract: In this paper, the desired performance of the mechanical system is specified in terms of contouring error instead of traditional method which specifies a task as a desired timed trajectory tracking problem. By defining the task frame, a simplified contouring error model is obtained through projecting tracking error to this new frame. Then a novel adaptive contouring controller is developed directly in the task frame to handle bounded external disturbances and system model uncertainties while maintaining superior contouring tracking performance. The algorithm effectively exploit the the structure of manipulator dynamics to reduce the computation complexity. Experimental results on an AC motor driven X-Y table demonstrate the merit of significant improvement of the proposed controller for increasing contouring accuracy compared with other conventional control algorithms.

Grasping Force Optimization for Whole Hand Grasp

Abstract: In tasks of grasping and manipulation, the hand sometimes uses not only fingertips but also fingers' inner links and the palm to achieve more robust grasp. This kind of grasp is called whole hand grasp, or power grasp. One property of whole hand grasp is that the hand may not be able to generate grasping forces in any directions, so previous fingertip grasping analysis is no longer suitable for whole hand grasp. In this paper, concepts of active force and passive force are introduced. With these concepts, the contact force space is decomposed into four orthogonal subspaces. Considering the roles of both active force and passive force, a new cost index is proposed for the whole hand grasping force optimization, which is then reformulated into a convex optimization problem involving LMIs. Finally, numerical example and simulation results verify the validity and performance of our formulation of the problem with that new proposed cost index

Hybrid Automaton: A Better Model of Finger Gaits

Abstract: Large-scale motion of the grasped object is one of the tasks, which is involved in practical dextrous manipulation of multifingered robotic hand. When the large-scale motion can not be accomplished only by rolling and sliding of the finger, finger gaiting, or regrasping, is used. In this paper, two primitives of finger gaits are introduced. Based on the characteristic of finger gaits, we model finger gaits as a hybrid automaton. Finally, we do simulations on a three fingered hand to verify the validity of our model

Finite Motion Validation for Parallel Manipulators: A Differential Geometry Approach

Abstract: Type synthesis of low (3-5) degree of freedom (Dof) spatial parallel manipulators is well documented in literature. Recent approaches such as proposed in J.M. Herve and F. Sparacino (1991) - Z. Huang and Q.C. Li (2003) showed some systematic design capability, but did not develop an equally effective means to check for prescribed finite motion. In this paper, we studied the finite motion set of parallel manipulators from a general input-affine nonlinear system viewpoint. Differential geometry tools for controllability (reachability) analysis of nonlinear system on a differential manifold are utilized together with lie group theory. Our techniques are shown to be effective by applying to a systematic type synthesis method proposed in M. Jian, et al. (2005) and W. Yuanqing, et al. (2005)