Complete Failure Analysis of Attitude Determination and Control System

Abstract

This paper briefly explains the Attitude Determination and Control System of a 2U class nano-satellite. The primary sensors used for attitude determination are Sun Sensors and Magnetometer. The readings from these sensors, along with the values obtained from the International Geomagnetic Reference Field (IGRF) and sun model, are provided as inputs to the Quaternion Estimation (QUEST) algorithm. QUEST uses these values to calculate the optimum quaternion for obtaining the desired orientation. Then, the PD controller has been used for calculating the torque that needs to be applied to achieve the required orientation with the help of actuators (Magnetorquers and Reaction Wheel). However, in case of failure, if any of the sensors or actuators is unavailable, the system would not be able to provide the required accuracy for payload action. Hence, the requirement for different modes of ADCS by switching between sensors, actuators, or quaternion estimators was felt. This paper describes various possible failure cases and accordingly used ADCS modes. It schemes for different tolerance methods that can be used in the failure of one or more ADCS hardware. The paper is so structured that proper description of fault detection techniques for every sensor and actuator is done, which is later followed with a reconfiguration of ADCS onboard to overcome a failure autonomously. This paper also describes the implementation of a seven-state nonlinear filter, an Extended Kalman Filter (EKF) model which takes readings of Magnetometer and Gyroscope as input values, for quaternion estimation. The seven variable state vector consists of quaternion for aligning the body reference frame with the orbit reference frame, and the angular rate of the satellite concerning the inertial frame, and the six variable measurement vector consists of readings of Magnetometer and Gyroscope. The paper has an inclusion of a detailed explanation of the filter along with mathematical equations with the required series of graphs. Thus, this paper aims at explaining the controller as well as the hardware used in failure or eclipse conditions in nano-satellites where there is hardware limitation due to space environment constraints or failure and which gives the best possible pointing accuracy for the payload action.