A novel shape memory alloy-based filter wheel drive mechanism for astronomical payloads

Astronomical payloads for monitoring of X-rays from planetary bodies require an active filter wheel drive mechanism to control the energy threshold, encompassing a larger range of solar X-ray intensity. If the detector aperture in the monitoring instrument is small, a manageable count rate is obtained but with poor sensitivity during lower activity period. On the other hand, a detector sensitive for lower activity period does not provide meaningful data during large intensity periods, due to saturation of detector front-end electronics. A survey of the existing drive mechanisms revealed that motorized mechanisms have been used for such applications with inherent drawbacks of increased mass, higher cost, and electrical complexity. In the present work, authors have designed and developed a novel filter wheel drive mechanism that uses an eccentrically mounted pin actuated by sequential activation of three shape memory alloys wires together. Also, it consists of a compliant leaf spring that avoids the requirement of an additional hold down by providing detent to hold the filter wheel in its intended position and acts as prime mover when the torque developed by shape memory alloy wires gets diminished due to reduction in moment arm during wheel rotation. This design offers substantial reduction in mass, development cost, and electrical complexity that can be exploited to send multiple detectors on board future interplanetary spacecraft. This paper presents the details of design, analysis, hardware realization, testing, and novelty of the mechanism.