ALBA XALOC beamline diffractometer table skin concept design

Abstract

The XALOC beamline is dedicated to macromolecular crystallography. The position and angle of the X-ray beam at the sample change in horizontal and vertical directions. Consequently, the support of the sample has to follow the beam with two linear and two rotational movements. For this purpose ALBA engineering division has developed a positioning table in house. The main goal of the design was to achieve high resolution, accuracy and stability. Effective resolution and accuracy need adequate actuators and high stability, in the range of 0.1 µm and 0.2 µrad resolution and 1 µm repeatability and first resonance mode f0 = 45 Hz (static and dynamic), for a 1.5 m-long, 1 m-high and 0.6 m-wide table which can support 0.5 tonnes of payload. The actuators were chosen by analytical calculations, whereas the stability is accomplished by a design that minimizes the thermal drifts and avoids the amplification of the vibrations from the ground, increasing the frequency of the resonance modes. The concept is based on a big block of natural granite that acts as a very stable reference base, in terms of thermal expansion and stiffness, and the mechanics are placed as close as possible to this reference. In this manner, the thermal drift is minimized and the stiffness is maximized. Conceptually, the mechanics can be imagined as a skin around the granite base. To reach a proper design several solutions have been analysed, with Finite element analysis (FEA) tools, to validate a compromise solution which has been produced and tested.