BACKLASH, POSITIONING REPEATABILITY AND PRELOAD ANALYSIS OF A PARALLEL MECHANISM

Abstract

Given to the complexity of mathematical models in parallel mechanisms, a previous analysis of system characteristic errors is necessary. This paper determines backlash and positioning repeatability of a two rotary degrees of freedom parallel mechanism and analyses the needed preload to increase system accuracy. Firstly, the specialized bibliography in the design and characterization of mechanisms based on parallel kinematic is reviewed. In the majority of the analysed studies, errors such as backlash are absorbed by the kinematic model without characterizing components or analysing which is the most determining system errors. Subsequently, the methodology used for the characterization of the positioning system components is presented. To do this, procedures and devices used for each measuring system are described, and the different measurement systems such as optical lineal sensors, laser interferometer and inductive probes, are presented. Results obtained show that actuator backlash errors are around 45 m having an error deviation of 3 m. The application of a contraction preload on the spherical ball joint allows us to obtain a backlash lesser than 3 m and, therefore, to increase the positioning system accuracy. Finally, the two degrees of freedom parallel mechanism intended to be used in metrology applications and machining tool processes, has been characterized by means of a coordinate measuring machine. The mechanism characterization developed will allow us to determine geometric parameters in the mechanism calibration phase, as well as to develop, subsequently, correction procedures according to the needed preload to increase system accuracy.