Simulated and Experimental Kinematic Calibration of a 4 Degrees of Freedom Parallel Manipulator
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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.