Sharif Digital Repository

Airships are inherently sensitive to random atmospheric disturbances that could potentially make their data gathering and observation missions a formidable task. In this context robust closed loop feedback controllers are important. The present study is therefore focused on optimal feedback controller design of an indigenous domestically designed airship (DA) for added robustness against atmospheric disturbances. While the general airship six degrees of freedom (6DoF) governing equations of motion are mathematically nonlinear, one often needs to resort to local linearization methods to benefit from proven linear closed loop controller (CLC) design approaches. In this sense an optimal linear... 

Airships are inherently sensitive to random atmospheric disturbances that could potentially make their data gathering and observation missions a formidable task. In this context robust closed loop feedback controllers are important. The present study is therefore focused on optimal feedback controller design of an indigenous domestically designed airship (DA) for added robustness against atmospheric disturbances. While the general airship six degrees of freedom (6DoF) governing equations of motion are mathematically nonlinear, one often needs to resort to local linearization methods to benefit from proven linear closed loop controller (CLC) design approaches. In this sense an optimal linear... 

In this paper, a modified frequency control model is proposed, where the demand response (DR) control loop is added to the traditional load frequency control (LFC) model to improve the frequency regulation of the power system. One of the main obstacles for using DR in the frequency regulation is communication delay which exists in transferring data from control center to appliances. To overcome this issue, an adaptive delay compensator (ADC) is used in order to compensate the communication delay in the control loop. In this regard, a weighted combination of several vertex compensators, whose weights are updated according to the measured delay, is employed. Generating the phase lead is the... 

In this work, design of an observer-based nonlinear controller for a class of single input-single output (SISO) systems in the general strict feedback form subject to input saturation based on the back-stepping technique has been addressed. A full order adaptive observer is constructed to estimate the unknown states and system parameters simultaneously. The proposed control scheme is singularity free and can handle the input nonlinearity. Asymptotical stability of the closed-loop system in the presence of observer dynamics and input saturation has been established by the Lyapunov stability theorem. The effectiveness of the proposed controller is illustrated by applying the proposed scheme to... 

This paper presents fractional order multi-input multi-output (MIMO) controllers for the robust performance of airplane longitudinal motion. A novel necessary and sufficient criterion is offered by using the value set concept to analyze the robust performance of fractional order MIMO uncertain systems based on the location of the characteristic equation roots. The criterion is applicable to all linear time-invariant systems of commensurate and incommensurate orders with complex coefficients. The obtained results are applied to an uncertain linear model of a business airplane to improve the robust performance of its longitudinal motion by decentralized MIMO output feedback and MIMO state... 

In this paper, a new form of generalized nonsingular fast terminal sliding mode control approach is proposed to provide the finite time tracking in connected chain of double integrator nonlinear systems subjected to additive bounded unknown uncertainties, disturbances, and internal interactions. The proposed approach presents an adjustable finite time for achieving the tracking goal which is a summation of two separate tunable times including finite reaching time and finite settling time. Tuning of the total finite time is done by adjusting arbitrary parameters in the control inputs and sliding surfaces. The high frequency switching of the control method is removed by applying a second order... 

A method for controlling chaos when the mathematical model of the system is unknown is presented in this paper. The controller is designed by the pole placement algorithm which provides a linear feedback control method. For calculating the feedback gain, a neural network is used for identification of the system from which the Jacobian of the system in its fixed point can be approximated. The weights of the neural network are adjusted online by the gradient descent algorithm in which the difference between the system output and the network output is considered as the error to be decreased. The method is applied on both discrete-time and continuous-time systems. For continuous-time systems,... 

In this paper, two fractional-order linear controllers are proposed to stabilize unstable equilibrium points of a chaotic fractional-order system. The first controller is based on the dynamic output feedback control idea and requires detectability of the linearized model of the fractional-order system on the equilibrium point. The second controller is a dynamic state feedback controller and requires observability of the linearized model. In both considered cases, the stabilizability of the model is assumed. The number of inner states in the second controller is one and therefore its structure is much simpler than the first controller. To illustrate the applicability, these controllers are... 

This paper is devoted to the characterization of the surface defects using a recently developed AFM technique called frequency and force modulation AFM (FFM-AFM). The simulated system includes a recently developed gold coated AFM probe which interacts with a sample including single-atom vacancy and impurities. In order to examine the behavior of the above system on different transition metals, the molecular dynamics (MD) simulation with Sutton-Chen (SC) inter-atomic potential is used. In this study, an online imaging simulation of the probe and sample is performed, and the effects of the horizontal scan speed, the effective frequency set-point, the cantilever stiffness, the tip-sample rest... 

In this paper, we propose an optimal control technique for a class of continuous-time nonlinear systems. The key idea of the proposed approach is to parametrize continuous stale trajectories by sequences of a finite number of intermediate target states; namely, waypoint sequences. It is shown that the optimal control problem for transferring the state from one waypoint to the next is given an explicit-form suboptimal solution, by means of linear approximation. Thus the original continuous-time nonlinear control problem reduces to a finite-dimensional optimization problem of waypoint sequences. Any efficient numerical optimization method, such as the interior-reflection Newton method, can be... 

In this paper, an adaptive control scheme, based on fuzzy logic systems, for pH control is addressed. For implementation of the proposed scheme no composition measurement is required. Stability of the closed-loop system is established and it is shown that the solution of the closed-loop system is uniformly ultimately bounded and under a certain condition, asymptotical stability is achieved. Effectiveness of the proposed controller is tested through simulation and experimental studies. Results indicate that the proposed controller has good performances in set-point tracking and load rejection and much better than that of a tuned PI controller. © 2009 Elsevier Ltd. All rights reserved  

The Endurance Time (ET) method is a new dynamic pushover procedure in which structures are subjected to gradually intensifying acceleration functions and their performance is assessed based on the length of the time interval that they can satisfy required performance objectives. In this paper, the accuracy of the Endurance Time method in estimating average deformation demands of low and medium rise steel frames using ETASOf series of ET acceleration functions has been investigated. The precision of the ET method in predicting the response of steel frames in nonlinear analysis is investigated by considering a simple set of moment-resisting frames. An elastic-perfectly-plastic material model... 

Fuzzy sliding mode control (FSMC) as a robust and intelligent nonlinear control technique is proposed to control processes with severe nonlinearity and unknown models. The performance of the proposed method has been evaluated for both single input single output (SISO) and MIMO nonlinear systems through its application in three severely nonlinear processes that are frequently used as benchmarks of nonlinear process control strategies. The evaluation shows that, despite its lack of dependence on the process model, the proposed method performs almost the same as conventional sliding mode control alternatives that utilize all the information that exists in the mathematical model of the process.... 

The biped walking robot demonstrates a stable limit cycle on shallow slopes. In previous researches, this passive gait was shown to be sensitive to ground slope and initial conditions. In this paper, we discuss the feedback stabilization of a biped robot by the "energy shaping" technique. Two designs are proposed to reduce the sensitivity of the biped walking robot to slope and initial conditions. In the first design, a moving mass actuator is located on each link of the robot. The actuators are used to shape the potential energy of the biped robot so that it tracks the potential energy of a known passive gait of a similar biped robot on a different slope. Although the method is applied to a... 

The taping mode Atomic Force Microscopic (T-AFM) can be assumed as a cantilever beam which its base is excited by a sinusoidal force and nonlinear potential interaction with sample. Thus the cantilever may cause chaotic behavior which decreases the performance of the sample topography. In order to modeling, using the galerkin method, the PDE equation is reduced to a single ODE equation which properly describing the continuous beam. In this paper a nonlinear delayed feedback control is proposed to control chaos in T-AFM system. Assuming model parameters uncertainties, the first order Unstable Periodic Orbits (UPOs) of the system is stabilized using the sliding nonlinear delayed feedback... 

The current paper focuses on the non-linear torsional vibration of a one-stage transmission gear system. Four different methods have been applied for solution of the equation of motion; the discretization method, the perturbation method, the Ritz method, and the stepwise time integration of the equation of motion. The time and frequency results from these analyses have been compared with each other, as well as those reported in literatures. Although all of these methods are accurate and computationally effective for finding the main spectral contribution, however, only the discretization method and the step-wise time-integration model are able to identify the other frequency components. ©... 

In this paper, an adaptive multi-model sliding mode controller for robotic manipulators is presented. By using the multiple models technique, the nominal part of the control signal is constructed according to the most appropriate model at different environments. Adaptive single-input-single-output (SISO) fuzzy systems or radial basis function (RBF) neural networks, regarding their functional equivalence property, are used to approximate the discontinuous part of control signal; control gain, in a classical sliding mode controller. The key feature of this scheme is that prior knowledge of the system uncertainties is not required to guarantee the stability. Also the chattering phenomenon in... 

The main purpose of this study is to compare identical synchronization and linear generalized synchronization from various points of views including synchronization error variance, convergence time, control effort variance, maximum and minimum values of the control signal. The nonlinear control is used in the both synchronizations. The results of our study show that the identical synchronization is not the optimal one from the mentioned criteria points of view. © ICROS  

Purpose - This paper aims to develop an adaptive unscented Kalman filter (AUKF) formulation for orientation estimation of aircraft and UAV utilizing low-cost attitude and heading reference systems (AHRS). Design/methodology/approach - A recursive least-square algorithm with exponential age weighting in time is utilized for estimation of the unknown inputs. The proposed AUKF tunes its measurement covariance to yield optimal performance. Owing to nonlinear nature of the dynamic model as well as the measurement equations, an unscented Kalman filter (UKF) is chosen against the extended Kalman filter, due to its better performance characteristics. The unscented transformation of the UKF is shown... 

Controlling nonlinear systems have always been a challenging problem. Complexity is mostly the result of nonlinear systems behavior dependence on initial conditions and input. Linearization techniques are such kinds of nonlinear systems analysis tools, which can give suitable results in neighborhood of equilibrium points. In addition, phase portraits are very efficient for visualizing the behavior of system in equilibrium points' neighborhood. In this paper, designing a controller for a low order dynamic system with multiple equilibrium points in presence of a sinusoidal disturbance with unknown amplitude and unknown bounded frequency is investigated. The proposed controller is based on a...