Applied nonlinear control of spacecraft simulator with constraints on torque and momentum of reaction wheels.

In this paper, the attitude of a spacecraft simulator is controlled in the presence of nonlinearities, uncertainties and constraints of the system. An optimization-based controller is developed in the closed form based on predicting the nonlinear responses of spacecraft system. For practical requirements of the spacecraft actuated by the reaction wheels (RWs), the control method considers the actuator limitation in the torques applied to RWs and the restriction in the angular momentum of the wheels due to the limited rotational speed of motors. In this way, the momentum constraint is equated with the torque constraint by a prediction-based transformation. Then, the constrained optimal control law is calculated by solving an equivalent optimization formulation. The constrained stability is analyzed by the Lyapunov second method and the bound of system response is obtained in terms of the prediction time. In the results, at first, the performance of the unconstrained version of the proposed controller, which leads to the feedback linearization like controller, is evaluated in the presence of uncertainties through computer simulations. Finally, the constrained version is simulated and experimentally implemented as hardware in the loop on the spacecraft simulator. The obtained comparative results indicate a good performance for the constrained controller in realistic conditions.

Practical fractional-order nonsingular terminal sliding mode control of spacecraft.

In this study, a new adaptive fractional-order nonsingular terminal sliding mode (AFONTSM) controller is presented. A novel multi-purpose sliding surface is constructed, with the aim of bringing the reaction wheels in to rest after every attitude stabilization maneuver, utilizing the fractional-order difference of the quaternion error and the reaction wheels angular momentum error. The closed-loop system's practical fixed-time stability is investigated using the Lyapunov theorem under uncertainty and external disturbance. The AFONTSM controller's performance is compared with the existing nonsingular terminal sliding mode (NTSM), full-order NTSM, and fractional-order sliding mode controllers. Finally, the proposed AFONTSM controller's effectiveness is studied in close-to-reality situations through practical experiments on the spacecraft attitude control subsystem simulator under internal/external disturbance and uncertainty; then, the results are compared with previous studies.

Classification and quantification of human error in air traffic control: a case study in an airport control tower.

This study aims at exploring human error in an airport control tower through the technique for the retrospective and predictive analysis of cognitive error (TRACEr) and the controller action reliability assessment (CARA) method. Despite the presence of automated safety nets, air traffic control (ATC) is heavily dependent upon the capabilities of humans. A number of ATC-relevant accidents were characterized by human errors. The data related to error dimensions were collected through interview and direct observation. Then, human error probability and error-producing conditions were evaluated by the CARA method. The results showed that selection and quality, memory, distraction/preoccupation, and traffic and airspace have the highest percentage error rates. Furthermore, the results indicated that the highest probability of error was associated with emergency situation management. This study is the first research to classify and quantify human errors using the TRACEr and the CARA method to evaluate controller error in ATC.

Chiral separation of three agrochemical toxins enantiomers by high-performance liquid chromatography on a vancomycin crystalline degradation products-chiral stationary phase.

This work aims to evaluate for the enantiomeric separations of three agrochemical toxins: haloxyfop-methyl, fenoxaprop-p-ethyl and indoxacarb on crystalline degradation products-chiral stationary phase (CDP-CSP) of high-performance liquid chromatography (HPLC) under normal and polar organic phases. In the normal phase, the mobile phase was n-hexane with alcohols including methanol and isopropanol as polar modifiers. In the polar organic phase mode, the mobile phase was methanol with different percentages of triethylammunium acetate. The influence of flow rate (0.3-0.9 mL/min), analyte concentration and silica gel particle sizes (10, 15 and 30 microm) was investigated. This new chiral stationary phase showed excellent stereoselectivity for the two enantiomers of haloxyfop-methyl and fenoxaprop-p-ethyl and chiral recognition for indoxacarb under normal-phase mode. However, under polar organic phase, only indoxacarb was separated (alpha < 1.5). The chromatographic results were compared with commercial chiral columns.