A Cryogenic Positioning System for Linear Variable Filter Optical Characterization
J. Hooftman (TU Delft - Mechanical Engineering)
N. Bhattacharya – Mentor
W. Jellema – Mentor (SRON–Netherlands Institute for Space Research)
Martin Eggens – Mentor (SRON–Netherlands Institute for Space Research)
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Abstract
PRobe far-Infrared Mission for Astrophysics (PRIMA) is a cryogenically cooled NASA mission aimed at making hyperspectral observations in the far infrared. The Netherlands Institute for Space Research (SRON) will design and manufacture a Linear Variable Filter (LVF) for use within PRIMA’s optics. In the design and manufacture of the LVF it is essential to characterize its optical properties. This is done by scanning a laser beam over the LVF’s surface while at 4.2K within a cryostat by means of a cryogenic positioning system. This research is tasked with designing that cryogenic positioning system; which must be capable of 2 degree-of-freedom planar motion to scan the the 40 × 20 mm surface of the LVF, with steps of 25 µm and an uncertainty in LVF position of ±2.5 µm. Additionally, the mechanism must fit within the cryostat, and must be able to bring the LVF to a position that is out of the laser beam, resulting in a total horizontal range of motion of 57.5mm. Due to the large range of motion, high accuracy requirements, low temperature, and limited available space, off-the-shelf solutions are not viable. A custom, highly integrated, solution must be developed. The far-infrared and cryogenic context requires that heat dissipation is kept to a minimum, and has consequences for the applicability
of standard components such as ball bearings.
The design process begins with divergent concept generation. Through the course of two downselects, concepts of lesser promise are eliminated. The final concept consists of a 5-bar mechanism with a parallelogram to constrain the rotation of the LVF. It is driven by two stepper motors. Through the use of a MATLAB model, the lengths of the links that define the mechanism are optimized. A Monte Carlo tolerance analysis is performed to calculate the expected performance of the manufactured mechanism. Lastly, the theoretical mechanism is materialized as a CAD model.
The final design of the mechanism is capable of scanning the 40 × 20 mm surface of the LVF with steps of 25 µm or smaller; meeting the target requirement. The worst-case position uncertainty of of the LVF is approximately ±5 µm; twice larger than the target requirement. The estimated heat dissipation is twice the targeted requirement, at 19.4 mW. The detent torque of the recommended stepper motor-gearbox combination provides a safety factor of 12.6 over the maximum torque required by the mechanism.