Calibration Method and Uncertainty Propagation Methodology for a Portable Parallel Kinematic Machine Tool

Autor: Aitor Olarra Urberuaga Director tesis Dragos Axinte, University of Nottingham Fecha2019

he in-situ maintenance and repair of goods that are in service rely on large sets of specialised tools that most of the times are of single use. The manufacture of features in large parts during setup also suffers the same lack of versatile tooling. The portable machine concept aims to fill this gap.

However, the feasibility of performing in-situ machining-like operations poses several challenges such as accessing the place of intervention, fixating of the machine to the part, referencing the coordinate systems or the variety of operations that are required to perform.

In this research, a self-portable parallel kinematic machine is proposed to perform in-situ machining interventions that can walk to the place of intervention. The parallel kinematics were extensively explored for machine tool applications between 1995 and 2005 but were almost abandoned as all-purpose machine topology due to difficulties related to design and limited workspace and accuracy. However, the high stiffness to mass ratio inherent to parallel kinematics machines is a crucial factor in building in-situ machining solutions on top of them.

The thesis covers the modelling that supports the analysis of the workspace, the estimation of the actuator forces and velocities and the stiffness at the tool centre point. The preliminary results of walking and machining trials that demonstrate the feasibility of the concept are presented.

Uncertainty propagation calculations extensively support the kinematic calibration solution design methodology. The methodology, adopted from the regression theory, has demonstrated to be a powerful design tool to predict the positioning performance of the machine. The uncertainty estimations have been validated experimentally by carrying out 36 calibrations in different configurations and analysing the variance of those parameters that should be constant. The agreement between uncertainty estimation and experimental variance have been very good.

Therefore, it can be stated that the work done on this thesis represents an advance in the methodology for the design of calibration solutions, having been successfully applied to a novel concept of portable machine.

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