سید مجید اسماعیل زاده

In this paper, a novel output redefinition is presented for attitude control of a flexible spacecraft. Attitude maneuvers induce undesirable vibration in the flexible appendages which degrades attitude control system performance. The attitude pointing and stabilization requirements of today advance space missions show the undesirable vibration suppression importance. If the flexible appendage tip-point is selected as the system output, the system model is nonminimum phase. The output redefinition method is an effective method to make the model minimum phase. By a deeper insight about the output redefinition method, a novel systematic method is presented such that the attitude control system performance is modified. Then, a nonlinear H∞ attitude control method is desined based on the redefined model system. For the situation that modal variable are not measurable, a modal observer is presented. The simulation results verify the presented method effectiveness in a flexible spacecraft attitude control problem in presence of mode uncertainties and unknown disturbance.

This paper addresses the problem of fixed-time control for a class of second-order nonlinear systems in the presence of model uncertainty and external disturbance. By introducing a novel form of non-singular terminal sliding mode control, a fixed-time control is designed to obtain acceptable performance, rapid convergence of the system states, high robustness and singularity elimination. Guaranteeing fixed-time convergence is a significant feature of the proposed control law under which the convergence time of the proposed surface is independent of the initial conditions. Since the upper bound of the system uncertainty and disturbance is quite difficult to obtain, an adaptive mechanism is presented under which there is no need to know this upper bound. Lyapunov analysis proves that the system states converge to small neighborhood of the origin within a fixed time. To assess efficiency of the suggested method, a flexible spacecraft attitude control system is considered and a fixed-time attitude control system is derived. Simulation results verify the effectiveness and performance of the presented approach.

This paper investigates the problem of finite-time stabilization of a class of uncertain nonlinear systems and a controller is proposed based on combination of integral sliding mode control with finite-time state feedback. The proposed controller consists of two parts. One part rejects matched uncertainties and the other part provides finite time stability. An adaption mechanism is also employed to estimate unknown parameters of the system. The proposed control law guarantees finite-time convergence of the sliding variable in the presence of uncertainties and unknown parameters. By elimination of the reaching phase, in which the system states are quite sensitive to any uncertainties or disturbances, the robustness of the system is guaranteed throughout the entire response. Furthermore, the upper bound of disturbance and uncertainties is not required to be known in advance which makes the suggested controller more flexible in terms of implementation.

Attitude Determining is one of the major and critical satellites space missions. In this study, a new method to Attitude determination of satellites is presented. Such that, based on the proposed method search space will be more limited then accuracy and speed of attitude determination in the proposed method has risen. At first in this method, implementation and the test algorithms will be discussed, after these some algorithms, such as navigation, pattern recognition and ultimately attitude determination will be reviewed. In order to implement these algorithm. High quality images of stars which must provided by the star tracker camera requires to implement. Really these images to perform the necessary processing sent to the processor so the processor based on designed algorithms, determines the attitude of camera and satellite in all three axes. This means that some features considered for star tracker and based on them begins the designing process. The range of accurately determination for star tracker is one of these features. In this article, the ranges of two axes of Yao and Pitch less than 20 seconds on the scale of degree are considered and in the roll axis less than 100 seconds is intended. Can show in the results, much better accuracy and less than initial assumptions have been achieved. It also carried out by an adaptive identified algorithm so that the brighter stars are identified and based on their attitude determination, the sensor accuracy have increased. Because of according research, the clearer stars, have more accurate in calculation. The other important feature is the speed of attitude detection which performed by 1 GHz processor, and correct identification of pyramidal algorithm where have reached less than 15 milliseconds. Due to the duration, the desire update rate gained. Other important parameters which influence the accuracy of the attitude determination is knowing the exact coordinates of the intersection point vector of focal length lens with image sensors. By Land calibration for camera with a good accuracy, these parameters were estimated.

A nonlinear model predictive control (NMPC) algorithm based on neural network is designed for boiler- turbine system. The boiler–turbine system presents a challenging control problem owing to its severe nonlinearity over a wide operation range¡ tight operating constraints on control move and strong coupling among variables. The nonlinear system is identified by MLP neural network and neural predictive control is designed by MIMO neural network model. Using neural network as predictive model in predictive control results in steady state error. A disturbance model is used in neural predictive model for elimination of steady state error and disturbance rejection. Simulation results on a boiler turbine system illustrate that a satisfactory closed-loop performance and offset-free property can be achieved by using the proposed method.

In this paper, the invasive weed optimization is used to determine the satellite orbit. The generated TLE can be used in antenna positioning, mission control and monitoring. Also, it can be sent to the satellite for on board applications. In the proposed method, the trial generated TLEs in each iteration are propagated to achieve the satellite position versus the time, and then they are compared with the real data position achieved from the tracking systems to build an error fitness function. Finally, the invasive weed optimization (IWO) algorithm is used to minimize this error. The proposed method is verified by using CHAMP satellite position data, as a real example.

In this paper, a simple new non-active RF switch is introduced to increase the reliability of the telecommand subsystem in a satellite. The telecommand subsystem manages the satellite mission by receiving the sent management commands and data from the ground station. These commands are received by the telecommand receiver and delivered to the onboard telecommand processor. The on board telecommand processor interprets and implements these commands, properly. The telecommand subsystem is designed and fabricated. Then the reliability analysis of this subsystem is studied by redundancy and improved by using the introduced non-active RF switch. The series switch reliability is very important due to using it as a series element. Therefore, a non-active switch with high reliability is designed and implemented. Low weight, low cost and simplicity are the main features of the proposed switch.

The new algorithm of simultaneously formation control and obstacle avoidance for quadrotors group is presented in this article. We consider the group of three quad rotors with nonlinear dynamic including rotors model in three dimensions space. Formation Control law in 3 dimensions was determined based on Euclidian distances and angles between quad rotors and controller designed with the direct control these parameters. Algorithm has ability of formation control and obstacle avoidance simultaneously. The presented algorithm reduces indirect measurement needs. Simulation results show algorithm ability in formation control and keeping, obstacle avoidance, smooth trajectory tracking and stability.The new algorithm of simultaneously formation control and obstacle avoidance for quadrotors group is presented in this article. We consider the group of three quad rotors with nonlinear dynamic including rotors model in three dimensions space. Formation Control law in 3 dimensions was determined based on Euclidian distances and angles between quad rotors and controller designed with the direct control these parameters. Algorithm has ability of formation control and obstacle avoidance simultaneously. The presented algorithm reduces indirect measurement needs. Simulation results show algorithm ability in formation control and keeping, obstacle avoidance, smooth trajectory tracking and stability.

In this paper predicting of position of satellite based on extended kalman filter with considering hardware implementation consideration and simultaneously maintaining desired accuracy is investigated. For this purpose, first, effective forces on orbital dynamic and nonlinear equation of orbital motion are presented. In order to increasing accuracy of prediction in position of satellite, J2, J3 and J4 harmonics of potential function of the earth are considered and future position of satellite is predicted using linearized dynamic model and applying EKF on this model. Here Measurement data are position and velocity vector of satellite which are extracted by GPS receivers. Since in this paper systematic satellite design is considered, scenario of “ON TIME” of GPS receivers based on power consumption considerations is discussed. Finally simulation results for a LEO satellite and comparing these results with STK results, shows accuracy of presented modeling and equations.

In this article by analyzing the TLE data of Navid satellite from Iran University of Science and Technology the effects of these data on tracking operation of ground station has been studied. Then the prediction accuracy of satellite position, the error rate of antenna ground station, the error rate of satellite rise time and the sensitivity survey of these toward the variation of drag term (􀜤∗) of TLE data has been investigated. The results show that the accuracy of TLE data, the intervals of updating them and the time elapsed the epoch related to TLE data directly effected on accuracy prediction of the satellite position and tracking operation. In some cases such as the limitation of ground antenna beam, tracking satellite is not succeeded. Therefore it has been suggested that tracking operation of satellite is done by ground station and independent of the TLE data and using TLE data as a reserve state has been considered. In this case TLE data must be updated daily.

The responsibility of the satellite thermal control system is to maintain equipments temperature in all external environments and under operational modes within an allowable temperature range. The geometric math model of satellite with available relations in references is obtained and certified with Thermal Desktop software. The outputs of geometric math model are external heating rates and radiation interchange factors. In this paper, the electrical simulation method is proposed as a tool for thermal math model of rotating satellite as equipments and structure of satellite are divided into several nodes and each term of thermal balance equation is simulated with equivalent electrical elements (capacitor, resistance, current source and etc.) and obtained circuit is solved fast and easily with HSPICE code. The values of voltage and current in each node are equivalent to temperature and heat flux, respectively. The results are illustrated the low run time with exact temperature responses of electrical simulation method in thermal modeling of satellite. By using the semi active thermal control, the thermal requirements are achieved and the effect of radiator paint is investigated.