This thesis discusses the kinematic calibration of the constraining linkage of a four degrees of freedom parallel manipulator. The manipulator has hybrid actuation of joints and wires, however the wires are not considered in this calibration. Two of the passive joints of the manipulator contain sensing so the calibration of the constraining linkage can be considered. Four kinematic models are developed for the manipulator. For each of these models, an independent set of model parameters are identified through an analysis of the augmented identification Jacobian matrix. Three different methods for formulating the augmented identification Jacobian matrix are explored. For the calibration, an optical tracking system is used to track the end-effector of the manipulator. The procedure to collect the calibration data is explained and the sources of error are considered. To further analyze the sources of error, simulated input data is created and the calibration using the experimental data and the simulated data are compared. In an attempt to improve the calibration, the selection of measured poses to be used for calibration is explored. Several different pose selection criteria have been proposed in the literature and five are evaluated in this work. The pose selection criteria were applied to the experimental manipulator and also a simulated two degrees of freedom manipulator. It is found that the pose selection criteria have a large impact when few poses are used; however the best results occur when a large number of poses are used for the calibration. An experimental calibration is carried out for the manipulator. Using the joint encoders and the kinematic model, the expected pose of the end-effector is calculated. The actual pose is measured using a vision tracking system and the difference between the actual and expected pose is minimized by adjusting the model parameters using a nonlinear optimization method. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-01-06 22:46:05.076

This study focuses on kinematic calibration of industrial robots. Kinematic modeling, parameter identification and optimal pose selection methods are presented. A computer simulation of the kinematic calibration is performed using generated measurements with normally distributed noise. Furthermore, kinematic calibration experiments are performed on an ABB IRB 6600 industrial robot using full pose measurements taken by a laser tracking system. The kinematic model of the robot is developed using the modified Denavit - Hartenberg convention. A nonlinear least-squares method is employed during the parameter identification stage. According to the experiment results, the initial robot positioning errors are reduced by more than 80%.

The aim of using parallel kinematic motion systems as an alternative of conventional machine tools for precision machining has raised the demands made on the accuracy of identification of the geometric parameters that are necessary for the kinematic transformation of the motion variables. The accuracy of a parallel manipulator is not only dependent upon an accurate control of its actuators but also upon a good knowledge of its geometrical characteristics. As the platform's controller determines the length of the actuators according to the nominal model, the resulted pose of the platform is inaccurate. One way to enhance platform accuracy is by kinematic calibration, a process by which the actual kinematic parameters are identified and then implemented to modify the kinematic model used by the controller. The first and most general valuation criterion for the actual calibration approaches is the relative improvement of the motion accuracy, eclipsing the other aspects to pay for it. The calibration outlay has been underestimated or even neglected for a long time. The scientific value of the calibration procedure is not only in direct proportion to the achieved accuracy, but also to the calibration effort. These demands become particularly stringent in case of the calibration of hexapods of the so-called simple design. The objectives of the here proposed new calibration procedure are based on the deficits mentioned above under the special requirements due to the circumstances of the simple design-concept. The main goals of the procedure can be summarized in obtaining the basics for an automated kinematic calibration procedure which works efficiently, quickly, effectively and possibly low-cost, all-in-one economically applied to the parallel kinematic machines. The problem will be approached systematically and taking step by step the necessary conclu-sions and measurements through: Systematical analysis of the workspace to determine the optimal measuring procedure, measurements with automated data acquisition and evaluation, simulated measurements based on the kinematic model of the structure and identifying the kinematic parameters using efficient optimization algorithms. The presented calibration has been successfully implemented and tested on the hexapod of simple design `Felix' available at the IWM, TU Dresden. The obtained results encourage the application of the procedure to other hexapod structures.

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