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Description
ANDES is a neutron diffraction instrument for materials characterization being developed by the Argentine National Atomic Energy Commission (CNEA) within the Argentine Laboratory of Neutron Beam(LAHN) project. Subsystem SS03 is responsible for orienting the neutron beam and selecting the incident neutron energy by positioning one of three monochromators within the beamline. Owing to the demanding requirements for positioning accuracy, repeatability, and mechanical stability, the subsystem was designed as two independent electromechanical machines.
The first machine, the Monochromator Shielding System (Blindaje de Monocromadores), is a two-axis mechanism that selects and positions one of two 320 kg neutron shielding blocks through vertical translation and rotary motion. Vertical displacement is provided by a pneumatic actuator, whereas rotational positioning is achieved through a four-stage gear transmission driven by a closed-loop stepper motor.
The second machine, the Monochromator Exchange System (Sistema de Intercambio de Monocromadores), is an eight-axis positioning system composed of three horizontal linear stages, one vertical linear stage, one rotary stage, and three circular-segment stages. This mechanism accurately positions and orients three neutron monochromators to direct the diffracted beam according to the experimental configuration. During operation, the selected monochromator is positioned through the coordinated motion of the corresponding horizontal stage together with the vertical, rotary, and circular-segment axes.
This work presents the verification methodology adopted during the development of SS03, combining mechanical metrology, tolerance analysis, manufacturing and assembly validation, and functional performance testing. For the Monochromator Exchange System, position repeatability tests were performed on the three horizontal linear stages to validate the positioning strategy and identify opportunities for design refinement.
For the Monochromator Shielding System, the verification process focused on achieving a positioning resolution better than 0.5 mm. The evaluation included geometric inspections of the rotating assembly, including flatness and parallelism runout measurements, as well as characterization of the gear transmission, with particular attention to the pitch diameter runout of the large driven gear. The overall positioning performance was assessed through cyclic accuracy and repeatability tests. The experimental results are presented and discussed together with the design modifications implemented to improve the subsystem performance.
The proposed verification methodology demonstrates the importance of integrating metrology, tolerance control, and experimental validation throughout the design process of precision mechatronic systems for neutron instrumentation, providing a systematic approach for achieving the positioning performance required by large-scale scientific facilities.
Key Words
shielding, tolerance, repeatability, accuracy, performance, metrology