Design & development of X-ray Solar Monitor (XSM) filter wheel drive mechanism with self hold down feature for chandrayaan-2 orbiter

Abstract

Onboard Chandrayaan-2 Orbiter, an X-ray Solar Monitor (XSM) instrument was flown for the measurement of intensity of solar X-rays within energy band of 1–15 keV. To achieve this, an active filter wheel drive mechanism is used to control the energy of the solar flares falling onto the detector by flipping the filter wheel. During normal flares, the filter wheel is steered to open position whereas during very large solar flares, a beryllium filter is positioned below the detector to reduce the incident solar energy. An in-built flare detection logic operates the mechanism in real-time when the threshold value of solar flares is crossed. The mechanism also has an additional calibration source which regularly calibrates the instrument.The filter wheel drive mechanism is developed as a compact, miniaturized system without requiring any hold-down thereby reducing the mass and design complications. The self-balanced filter wheel is designed by the ingenious usage of dead weights such that center of gravity of the wheel passes through the rotation axis. Together with the back-driving torque of the motor, the tendency of the wheel to jitter during launch is reduced, thus avoiding a separate hold down mechanism. An additional gripping force is ensured between the filter wheel and the motor shaft by using a grub screw. One end of the filter wheel shaft is supported at the base plate through a DU bush to reduce friction during filter wheel’s rotation and the other end is connected to the motor shaft. A novel Optical Sensor based wheel position feedback system is also developed for the position indication during pre and post dynamic tests.In this paper, the design details of the XSM Filter wheel mechanism along with its analysis and hardware performance evaluation during theintegrated tests with the Chandrayaan-2 Orbiter spacecraft is presented.