Sharif Digital Repository

In this research, the effect of variable stiffness and damping, in suspension system, on car performance was studied. The optimization of car performance was obtained by minimization of an objective function including the body acceleration, suspension travel and tire deflection. Firstly, considering the stochastic input for a quarter car model, minimization of the objective function was done by changing the stiffness and damping coefficients in the frequency domain. The Linear Quadratic Regulator (LQR) method was implemented to minimize the objective function. In order to supply the active force obtained by optimization, different semi-active designs, in which stiffness and damping were... 

Magnetic bearings have been introduced in recent decades to overcome the problem of energy loses due to friction in bearings of rotating machinery. In magnetic bearings, the rotor is suspended by magnetic forces without any friction. These bearings are of the two types of passive and active. In Passive Magnetic Bearings (PMB), magnetic forces are generated by permanent magnets and have the ability to tolerate a small distribution. In Active Magnetic Bearings (AMB) magnetic forces are generated by electromagnets and have the ability to tolerate larger distributions than the PMB can. Since AMB is inherently unstable, it is necessary to use controllers to stabilize it. Due to the nonlinear... 

Today, with improvements in industries such as the sensitive equipment’ industries and the optics industries which need great precision for their optimum performance, the need for a vibration-free environment is felt more than ever. The main objective in this thesis is the effective reduction of the effects of ambient vibrations on points on an optical concrete table using active control. In this research, other than separating the table with the passive method, the reduction of vibrations with active control and the LQR algorithm is investigated. In order to study the dynamic characteristics of the table and to observe the active control process better and more carefully, finite element... 

In this thesis the problem of spacecrafts formation control for halo orbit around the second libration point (L2) of the Sun-Earth Three Body (TB) system is investigated. Station keeping, reconfiguration and precision formation control of spacecrafts on halo orbits are performed via the use of the nonlinear Integral Sliding Mode (ISM) method, optimal closed loop Linear Quadratic regulator (LQR) approach as well as the Model Prodective Control (MPC). In this regard the nonlinear relative dynamics of deputy-chief spacecrafts are derived within the concept of both circular and elliptical three body problem; as well the perturbation model of the Moon and the linear model of restricted three... 

In this work the problem of optimal spacecrafts reconfiguration and formation keeping is investigated for Asteroid attraction. Due to potential impact probability of the Apophis asteroid with Earth in 2036, the study is focused on optimal intercept avoidance via gravity attraction of multiple spacecrafts suitably arranged on halo orbits of Apophis. In this regard, a space mission is defined in which initially several spacecrafts are optimally reconfigured onspecial Halo orbits of the asteroid via several techniques that include pseudospectral, linear quadratic regulator as well as the integral sliding mode control. The utility of three methods is for verification purposes as well as... 

In this research, the dynamics, aero-elasticity, and control of an airship with a flexible body hull and fins are investigated. The flexible airship is modeled as a combination of several beams with specific boundary conditions. So that the dynamics and vibration of its body hull and fins can be taken into account regarding the aerodynamic, gravity, aerostatic, and control input forces. The governing equations of motion of the system are derived by using the velocity of an arbitrary point on each component and integration over the component and substituting them into the Euler-Lagrange equations in a body frame. Afterward, the added mass acting on the hull is calculated. Then, a perturbation... 

Marine offshore structures are huge structures constructed in very harsh environments, like oceans and seas. These structures are highly exposed vibration due to ocean’s waves, wind, helicopter landing, etc. Preventing the damage to these structures due unpredicted vibration using a semi active vibration control is being considered. Magnetrorehological (MR) dampers have been used as active element to control the vibration and to provide the structural integrity of the structure. Providing high dynamic range, lower power requirements, large force capacity, robustness, and safe manner operation in case of fail made the MR dampers attractive devices to passive, semi-active and active control... 

In this work dynamic behavior of a two body TSS with main satellite as rigid body is studied and effects of tether elasticity and tension on the satellite dynamic are indicated in station keeping phase and circular orbit. Also a simplified modeling is introduced which could be used in nonlinear controller design. Lagrange and extended Lagrange method is used to derive equations of motion. It is shown that longitudinal vibration of tether has small effect on satellite attitude and angular velocity and because of high frequency it can be decoupled from satellite dynamic. In the simplified model, orbital motion is decoupled from equations of TSS and main satellite dynamic. Effect of decoupling... 

In the first part of the thesis, we introduce the structure of a Nano-Newton CMOS-MEMS capacitive force sensor. This force sensor has been utilized out-of-plane sensing mechanism with high accuracy and reliability. Also, we discuss about sensitivity analysis of this force sensor. The largest change of capacitance (increased capacitance + decreased capacitance) occurs in ℓ=2/3. Sensitivity for increased capacitance grows by 8.33% and for decreased capacitance grows by 2.27%. The proposed method provides useful guidelines to increase the sensitivity of the sensor up to 250%. In the second part, a fixed structure controller, PID type, is proposed to control and suppress the in-plane vibration... 

The problem of LQR design based on full state feedback is solved completely in the subject of Optimal Control. So far, however, despite the fact that use of output feedback in many applications of Control is desirable or even necessary, designing optimal controllers using output feedback in general does not seem to have received much attention in the existing literature. Of course, special cases of designing simple controllers using analytical or numerical-analytical methods do exist in the literature. A new algorithm for tuning an optimal controller (P, PI, PID, etc) for a low-order system without delay, and also in the presence of delay, has been introduced in this thesis using the LQR... 

The problem of spacecraft rendezvous on periodic orbits around the L_1Lagrange point of the Earth-Moon system is investigated. For this purpose, the nonlinear relative equations of motion are drived within the contex of the Circular Restricted Three Body (CRTB) problem. Subsequently, several types of rendezvous missionas well as control teqniques are considered for analysis and simulation. Application of different control approaches allows for partial verification as well as comparison of results achieved with various teqniques. The considered control approaches include optimal closed loop Linear Quadratic Regulator (LQR), nonlinear Feedback Linearazation (FL) as well as the nonlinear...