An Elegant Solution for a Deceptively Simple Filter Wheel Assembly

Abstract

As part of a larger undisclosed optical instrument, the Advanced Optical Systems Program Office of NASA's Jet Propulsion Laboratory requisitioned the support of Motiv Space Systems (Motiv) to design, analyze, fabricate and qualify a space flight Filter Wheel Assembly (FWA) to an extensive list of critical requirements. The full system suite of the FWA consists of a Filter Wheel Mechanism (FWM) with wheel structure of roughly 17″ diameter, fully redundant set of drive electronics (including motor and absolute output sensor) and all associated command software and cabling. The FWM leverages Motiv's harmonic gearbox based flight-qualified actuator design, while its space rated, rad hard dual axis BRAVO motor controller drive this actuator. Two boards have been implemented for a fully redundant system. As the general case with engineering design, determining driving requirements early on is of utmost importance. Such was the case in the development of the FWA. Several challenges presented themselves throughout the design process, and turned what at first blush appears to be a straightforward single degree of freedom mechanism, into a highly engineered system to meet the exacting needs of the overall project. Design aspects worth noting and described herein: 1) As part of the motor selection process and electronics architecture development it was determined that running the motor in a hybrid mode (utilizing both commutated and stepper modes) would solve power consumption and dissipation concerns while guaranteeing that optical element alignment requirements would be met. 2) Although not realized as a design driver initially, it became evident that a heat leakage requirement would significantly drive the final configuration of the actuator output shaft structure. The output shaft geometry pushed the limits of manufacturing capabilities, while special coatings (electroless nickel and gold) were implemented to further combat unwanted radiative heat transfer, but not without a few lessons learned along the way. 3) Furthermore, it resulted that the heat transfer requirement driving the output shaft design competed with a stiffness requirement, creating a compelling reason to select an exotic material (AlBeMet) for the large wheel structure. 4) Separately, a requirement for unique signatures for output position at each of the eight optical elements drove the need for the development of a custom output sensor. The final sensor configuration drew from prior projects at Motiv but took on a modified form factor to fit within the FWM architecture. All aspects combined, the full system FWA was able to meet the comprehensive needs of the program.