Control methods have been developed to remove the accumulated angular momentum in momentum wheels of spacecraft using two or one external control torques. Momentum wheels are used in spacecraft attitude control system to stabilize the desired attitude of the spacecraft consuming renewable electrical energy. The disturbance torques on the spacecraft increase the angular momentum of the spacecraft. This change results in speeding of the momentum wheels. The angular momentum stored in the momentum wheels should be removed by implementing external torques to prevent saturation of the momentum wheel motors. Control algorithms have been devised to remove the angular momentum in spite of loosing external torque control about one or even two of the principal axes of the spacecraft due to failures. The angular momentum of the corresponding momentum wheels is removed by performing attitude maneuvers. The spinning speed of the remaining wheels is then ceased using conventional momentum dumping methods. The attitude of the spacecraft is finally corrected to the desired direction. The proposed control laws are proven to be globally stable. Numerical simulations illustrate successful and smooth performance of the laws.

This paper presents a new scheme for fault detection and diagnosis in spacecraft Attitude Determination (AD) sensors. An integrated attitude determination system, which includes measurements of rate and angular position using rate gyros and vector sensors, is developed. Measurement data from all sensors are fused by a linearized Kalman filter, which is designed based on the system kinematics, to provide attitude estimation and the values of the gyro bias. Using this information the erroneous sensor measurements are corrected, and unbounded sensor measurement errors are avoided. The resulting bias-free data are used in the Fault Detection and Diagnosis (FDD) scheme. The FDD algorithm uses model-based state estimation, combining the information from the rotational dynamics and kinematics of a spacecraft with the sensor measurements to predict the future sensor outputs. Fault isolation is performed through Extended Kalman Filters (EKFs). The innovation sequences of EKFs are monitored by several statistical tests to detect the presence of a failure and to localize the failures in all AD sensors. The isolation procedure is developed in two phases. In the first phase, two EKFs are designed, which use subsets of measurements to provide state estimates and form residuals, which are used to verify the source of the fault. In the second phase of isolation, testing of multiple hypotheses is performed. The generalized likelihood ratio test is utilized to identify the faulty components. In the scheme developed in this paper a relatively small number of hypotheses is used, which results in faster isolation and highly distinguishable fault signatures. An important feature of the developed FDD scheme is that it can provide attitude estimations in presence of fault in one type of sensors.

Good robot performance often relies upon the selection of design parameters that lead to a well conditioned Jacobian or impedance "design" matrix. In this paper, a new design matrix normalization technique is presented to handle the problem of nonhomogeneous physical units and to provide a means of specifying a performance based design goal. The technique pre- and post-multiplies a design matrix by scaling matrices corresponding to a range of joint and task space variables. The task-space scale factors are used to set the relative required strength or speed along any axes of end point motion while the joint-space scale factors are treated as free design parameters to improve isotropy through nonhomogeneous actuation. The effect of scaling on actual designs is illustrated by a number of design examples using a global search method previously developed by the authors.

Good robot performance often relies upon the selection of design parameters that lead to a well conditioned Jacobian or impedance "design" matrix. In this paper a new design matrix normalization technique is presented to cope with the problem of nonhomogeneous physical units. The technique pre and post-multiplies a design matrix by diagonal scaling matrices corresponding to the range of joint and task space variables. In the case of the Jacobian, normalization leads to a practical interpretation of a robot's "characteristic length" as the desired ratio between maximum linear and angular force or velocity. The scale factors can also be used to set relative required strength or speed along any axes of end-point motion and/or can be treated as free design parameters to improve isotropy through asymmetric actuation. The effect of scaling on actual designs is illustrated by a number of design examples using a global search method previously developed by the authors.