This paper presents a robust sliding-mode control technique to be applied to quaternion-based attitude control for rest-to-rest maneuvers with external disturbances. A sliding mode controller has been designed to force the state variables of the closed loop system to converge to the desired values. A control strategy is designed based on a novel mathematical rule that computes the discontinuous feedback gains. The proposed approach is defined in such a way that the selected controller parameters can drive the state to hit the sliding surface fast and then keep the state sliding along the surface with less chattering and tracking error. Moreover, the control parameters are adjusted to avoid the body angular velocity reached the upper limit during the maneuver. A simulation model of the controlled spacecraft system was developed in MATLAB-SIMULINK software. The phase portraits and the state plots prove the control technique power. The "chattering" problem of the sliding mode control has been adopted using variable thickness boundary layer technique. The second method of Lyapunov is used to guarantee the system stability under the proposed control laws action. Simulations have been carried out to demonstrate and verify the developed controller performance.

Undersaturated oil viscosity is an important physical property for reservoir simulation, enhanced oil recovery, and optimal production. There are two distinct methods for undersaturated oil viscosity determination: the first one is experimental measurements which are usually expensive or unavailable; whereas the second one is empirical correlations which frequently have appropriate accuracy. Accordingly, searching for a high reliability undersaturated oil viscosity model is vital. This paper presents a new undersaturated crude oil viscosity model by using multi-gene genetic programming (MGGP). This model was built by using 528 experimental measurements data points that presents broad range of reservoir pressure and oil properties. Another, 276 points were used for validating and testing the new model against eleven published correlations. The results indicated that the new MGGP-based model yields a precise prediction of undersaturated oil viscosity.