Fault-Tolerant Attitude Control for Flexible Spacecraft with Input Saturation

In the past several decades, the attitude control problem of spacecraft has attracted a great deal of attention due to its important applications.1–3 As actuation devices generate toques with only limited amplitude and/or rate,4–6 input saturation may frequently occur during the entire attitude maneuvers of a spacecraft. As discussed in7,8 input saturation can severely degrade closed–loop system performance or even in some cases cause system instability if they are not carefully tackled in attitude control process. Because of this, many researchers have focused on input saturation in attitude controller design of spacecraft.9–13 In Hu,12 neural network was used to estimate the unknown input saturation and then the effect of input saturation was compensated by inserting the saturation compensator into the feed–forward loop of the system. In Hu,13 an auxiliary variable was employed to compensate the effect of input saturation in attitude controller design. However, the tracking error would be modified to get a stable control system with this auxiliary variable. In addition, during the spacecraft mission, actuators may undergo faults due to aging or accidents, such as partial loss of effectiveness, stuck and outage. These faults may cause system instability or even end up with catastrophic events if they are not well handled. Therefore, designing a controller that is robust to these actuator faults is one of the significant issues that need to be considered by researchers. Fault–tolerant control (FTC)14,15 is considered as one of the most effective approaches for maintaining stability and expected performance of system during the actuator faults occurrence. More and more literatures have focused on fault–tolerant attitude control for a spacecraft; see.16–22 In Jiang,21 the authors proposed fault–tolerant attitude control of spacecraft to accommodate the partial loss of effectiveness faults in actuators with a prior knowledge of the lower bound of the effectiveness factor. In Hu,22 the knowledge of the bound of the partial effectiveness factor was not need by employing an update law to estimate the lower bound online. However, these mentioned literatures above less considered another important fault scenario of actuators, i.e., stuck fault. As far as we know, in attitude control design, there are few literatures that take stuck fault into account explicitly expect for Hu.23 But in Hu,23 input saturation were not considered, which might be sometimes conservative in practical applications.