Computational Methods in Engineering

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In this paper, a new method is proposed for the speed control of a three-phase current forced synchronous machine drive. This metohd is based on the use of an approximate constant commutation safety margin angle in the output thyristor bridge of the drive system as well as on the compenastion of stator magnetization reaction. The commutation process in the output converter of the machine drive is naturally performed by means of the rotor induced emfs in the stator winding. In comparsion with other speed control methods reported for these types of drives, this method is more stable and robust. In additon, higher efficency and power factor can be achieved for the synchronous machine drive through this method. The impacts of rotor damper windings on the drive system performance are also explained in the paper.

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In this paper, a new method based on genetic-fuzzy algorithm for multi-objective optimization is proposed. This method is successfully applied to several multi-objective optimization problems. Two examples are presented: the first example is the optimization of two nonlinear mathematical functions and the second one is the design of PI controller for control of an induction motor drive supplied by Current-Source-Inverter (CSI). Step response of the system is considered and controller parameters are designed based on multi-objective optimization technique. Rise-time, maximum over-shoot, settling time and steady state error are considered as objective functions. The simulation results of the new method for induction motor speed control and optimization of two nonlinear mathematical functions are compared with the results obtained from other methods [4,14,15], which shows better performance.

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Finding the collapse susceptible portion of a power system is one of the purposes of voltage stability analysis. This part which is a voltage control area is called the voltage weak area. Determining the weak area and adjecent voltage control areas has special importance in the improvement of voltage stability. Designing an on-line corrective control requires the voltage weak area to be determined by a sufficiently rapid and precise method. In this paper, a new algorithm based on assigning a vector to each power system bus is presented. These vectors indicate buses conditions from the viewpoint of voltage stability. In this new method, using the clustering methods such as kohonen neural network, fuzzy C-Means algorithm and fuzzy kohonen algorithm, voltage control areas are determined The proposed method has advantages such as determining PV and PQ buses which belong to the weak area simultanously, under all operating conditions and without a need to system model. Also by comparing the results of applying clustering methods, it has been observed that, due to simplicity of implementation and precision of the results, the two dimensional kohonen neural network is a more suitable tool for clustering power system to voltage control areas than the fuzzy C-Means and fuzzy kohonen methods. Keywords: Voltage stability, Voltage weak area, Voltage control area, Corrective control, Pattern recognition, Kohonen neural network, Fuzzy C-Means algorithm, Fuzzy Kohonen algorithm.

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Because of extreme local saturation at pole tips of excited phase and uncircular shape of rotor and stator, a Swithed Reluctance Motor (SRM) does not have a simple and accurate mathematical model. Therefore, the output control of this motor requires a robust controller which is not based on an accurate model of the process. Fuzzy controllers, to some extent, will satisfy these requirements. Teta-on and teta-off are controller outputs. The output of teta-off controller is a Variable Structure Controller (VSC) which contains two parts: coarse controller which is used when the speed error is large and its output causes large changes in teta-on angle. This part of the controller is similar to a fuzzy PI controller. The other part of the controller is a fine controller and is used when the speed error is low. The fine controller increases the speed of response and reduces the speed error to zero. This part is similar to a fuzzy I or PI controller. Finally, experimental results of no-load and underload speed controls are demonstrated. The fuzzy controller robustness to measurement noise and parameter uncertainty is also studied. Keywords: Fuzzy Controller. SRM Variable Structure Controller

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In most stochastic inventory models, such as continuous review models and periodic review models, it has been assumed that the stockout period during a cycle is small enough to be neglected so that the average number of cycles per year can be approximated as D/Q, where D is the average annual demand and Q is the order quantity. This assumption makes the problem more tactable, but it should not be adopted when the beck order and lost sales penalty costs are relatively small. In this paper, considering a continuous review inventory model, we relax the above assumption and we explicitly take into account the stockout period when computing the expected cycle length. Further, we consider the effect of using exact number of cycles rather than using approximate of cycles in a continuous review inventory model. Keywords: Inventory control, Stochastic demand, Continuous review, Inventory cycle

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Various methods are presented to control the attitude of satellite due to lack of information of yaw axis attitude so far. In this paper, a new method is used which is more general and is applicable to all types of satellites, with at least one controller. This method is not limited to YAW axis and can control the two other axes as well. One feature of this approach is the ability to be modified for large angle maneuvers for special applications. A new observer is also proposed for the controller, which causes the angles attitude to be kept in desirable values, with minimum control effort and more accuracy. Simulation results show that compare with methods using the complete all axis information, this approach is satisfactory. Keywords: Attitude control, PID controlley, BIBO stability, pseudo observer

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In recent years many different ways have been investigated by steel producers to increase ductility, impact strength and formability of steels. More important ways are steels with very low amounts of inclusion, small size inclusions and modified inclusions. In this study, experiments have been performed on the API-X42 steel produced by the electric arc furnace in Mobarekeh Steel Co. After the preparation of the melt in the electric arc furnace, it is taped in a 200-ton ladle and ladle treatment was preformed in a ladle furnace (LF) in the steelmaking shop. In this study the effects of amount and the rate of CaSi wire addition on the shape and structure of inclusion were investigated. The optimum conditions for adding CaSi for inclusion shape control were also determined. Scanning electron microscope (SEM) and chemical energy analysis dispersive system (EDS) showed that adding calcium to the melt affects the chemical composition of inclusions present in steel melt. The effects of CaSi wire injection treatment, injection amount and inclusion shape control on the impact property and formability of steel were shown using charpy impact test. Keywords: Inclusion shape control, calcium treatment, ladle furnace and inclusion modification

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In this paper, using a personal computer (PC), the practical implementation of scalar and vector control methods on a three–phase rotor surface- type permanent magnet synchronous machine drive is discussed. Based on the machine dynamic equations and the above control strategies, two block diagrams are presented first for closed-loop speed controlling of the machine drive/system. Then, the design and implementation of hardware circuits for power, insulating, and signal matching stages are explained along with a description of the written software program for the servo drive system control. These circuits are used to produce the drive inverter switching pulses. To supply the machine drive, the sinusoidal, uniform sampling and step-trapezoidal PWM voltage source inverters are examined. For closed loop speed control of the drive system, the stator currents and rotor speed signals (in scalar control method only the rotor speed) are sampled on-line. After filtering, buffering and matching operations, these signals are transferred to a personal computer port via a high frequency sampling and high resolution A/D converter. It is worth mensioning that both methods of controlling mathematical calculations is done by computer. Finally, the practical and computer simulation results obtained are demonstrated. Keywords: Machine Drive, Synchronous Machine, Permanent Magnet, Rotor Surface Type, Scalar and Vector Control, Voltage – Source Inverter, Control by PC.

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Soft Switcing techniques have recently been applied in the design of dc-ac converters, in order to achive better performance, higher efficiency, and power density. One of the soft switching techniques uesd in inverters is resonant dc links. These topologies have some disadvantages such as irregular current peaks, large voltage peaks, uncotrollble pulse width, etc. Another soft switching method in inverters is using Quasi –resonant links, which have PWM modulation capability. Inverters with series or parallel Quasi-resonant dc links use several quasi-resonant current or voltage pulses, respectively, to produce PWM modualation. In this paper an inverter with a novel Quasi-resonant series dc link is introduced. This topology enables current source inverters to have characteristics such as resonant pulse peak limition and pulse width controllability. This circuit provides the inverter with two to three ranges of PWM control capability which increases the switching time control in a larger range. Various operational modes of this novel Quasi-resonant dc link is analyesed and then the circuit losses is calculated. Finally, simulation results by PSPICE software is presented to justify the circuit operation. Keyword: Inverter, Soft switching, Novel quasi-series resonant link, increasing control areas, Losses

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Designing a robust tracking control for a non-linear MIMO system with uncertainty is one of the most complicated control problems. In this paper, sliding mode changed to non-linear controllable canonical form by input-output linearization. This, sliding surfaces can be defined in a way that we can de-couple equations and indicate the sliding conditions of multi-variable controller system. The uncertain parameters will be estimated properly and the input equation improved to apply the restricted input condition. The control law will be improved in a way that in addition to increasing the tracking accuracy inside the boundary layer, the speed of convergence will increase outside of the boundary layer. In order to satisfy the balance of the filter, the thickness of the adaptive boundary layer is used. Thus, a robust tracking control is designed which can trace the angle of attitude of satellite for maneuvers with a very large angle (180 deg.) on a piece-wise smooth path. Finally, the simulation results are compared with Elmali & Olgac’s methods and it is shown that despite decreasing control signals, the tracking accuracy increases by several ten times. Keywords: Attitude, Control, Sliding-mode control, Non-linear System, Input-Output Linearzation

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In this paper, based on feedback linearization control method and using a special PI (propotational integrator) regulator (IP) in combination with a feed-forward controller, a three-phase induction servo-drive is speed controlled. First, an observer is employed to estimate the rotor d and q axis flux components. Then, two input-output state variables are introduced to control the dynamics of torque and the magnitude of the rotor flux independently. In addition, based on the model refrence adaptive system (MRAS) and the recursive least square (RLC) error techniques, the rotor time constant and the mechnical parameters (J, R) are simultaneously estimated. Finally, the efficiency of the proposed method is confirmed against results from computer simulation. Keywords: Adaptive speed ontrol, Inducation servo-drive, Feedback linearization, IP controller, Model reference, Adaptive system, Recursive least square.

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The objective of this study is to design a robust direct model reference adaptive controller (DMRAC) for a nonlinear cardiovascular model over a range of plant parameters representing a variety of physical conditions. The direct adaptive controllers used in thisd study require the plant to be almost strictly positive real (ASPR) that is, for a plant to be controlled there must exist a feedback gain such that the resulting closed loop system is strictly positive real. We designed a new compensator so that the system composed of the cardiovascular plant and the compensator satisfy the ASPR condition. Numerous studies in the past have considered a small range of gain variations of the cardiovascular system. In most cases, the controller was designed based on variations in either time delay or plant gains. Many of these workers treated the cardiovascular system as a single-input single output (SISo) plant in which the control output was Mean Arterial Pressure (MAO). We treated the cardiovascular system as a multi-input multi-output (MIMO) plant in which both the MAP and Cardiac Output (CO) are simultaneously controlled. In this study, a new linear model is presented that provides a better approximation thanthe one the original linear model does. By doing so and utilizing the DMRAC algorithm, we could satisfy the stability conditions for the nonlinear model while satisfactory responses obtained under every possible condition for the cardiovascular nonlinear model. Keywords: Adaptive control, Cardiovascular system, Blood pressure, Cardiac output

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This study is an attempt to introduce scientific fundamentals and available methods for wellhead protection area (capture zone) delineation for drinking water wells in cities. The results of this study could obviate some demands of the national water and wastewater company in quality control of the drinking water resources by delineation and application of the wellhead protection areas. For this purpose, the available literaturer reviewed to extract, criteria and methods of wellhead protection delineation, Then, (1) fixed radius method, (2) simplified variable shape methods, and (3) flow-transport analytical methods implemented in the computer code WHPA are introduced. The applicability of these methods is shown by some sample calculations for Urmia drinking water wells. Samples of the calculated wellhead protection areas for 36 wells in Urmia City will be shown using three analytical modules in WHPA. The effects of the hydrogeologic parameters on the wellhead areas will be discussed. When reliable hydrogeologic parameters are available in the region where wells are located, the analytical methods and WHPA code produce accurate results for wellhead protection areas.

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Stator flux oriented vector control of induction motor (IM) drives for speed sensorless control has several advantages. But the application of a pure integrator for the flux estimation is difficult due to the presence of measurement noise and dc offset. To overcome these problems, some have used a programmable cascaded low pass filter (PCLPF). In this paper, it is shown that some problems still exist and some new problems arise from this approach. In order to solve these problems, a novel compensation method is proposed. In this scheme, the dc offset is detected and subtracted from the estimated flux along d and q axes. The simulation results show that it works well in the low speed region as well as in the transient state. The oscillation of the torque and the estimated flux are also reduced notably when the torque reference changes rapidly.

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In the last few years, Run-to-Run (R2R) control techniques have been developed and used to control various processes in industries. These techniques combine response surface, statistical process control, and feedback control techniques. The R2R controller consists of a linear regression model that relates input variables to output variables using Exponentially Weighted Moving Average (EWMA). In this paper, we have developed a R2R controller model based on quality costs. The model consists of finding optimum weight of EWMA procedure in R2R controllers with respect to conformities and nonconformities costs. The validity and performance of the developed model were tested using a real case study in an optic industry application.

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The rapid growth of space utilization requires extensive construction, and maintenance of space structures and satellites in orbit. This will, in turn, substantiate application of robotic systems in space. In this paper, a near-minimum-time optimal control law is developed for a rigid space platform with flexible links during an orientating maneuver with large angle of rotation. The time optimal control solution for the rigid-body mode is obtained as a bang-bang function and applied to the flexible system after smoothening the control inputs to avoid stimulation of the flexible modes. This will also reflect practical limitations in exerting bang-bang actuator forces/torques, due to delays and non-zero time constants of existing actuation elements. The smoothness of the input command is obtained by reshaping its profile based on consideration of additional first-order and second-order derivative constraints. The platform is modeled as a linear undamped elastic system that yields an appropriate model for the analysis of planar rotational maneuvers. The developed control law is applied on a given satellite during a slewing maneuver. The simulation results show that the modified realistic optimal input compared to the bang-bang solution agrees well with the practical limitations and also alleviates the vibrating motion of the flexible appendage, which reveals the merits of the new control law developed here.

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Mobile robotic systems, which include a mobile platform with one or more manipulators, mounted at specific locations on the mobile base, are of great interest in a number of applications. In this paper, after thorough kinematic studies on the platform and manipulator motions, a systematic methodology will be presented to obtain the dynamic equations for such systems without violating the base nonholonomic constraints. Combining the kinematic model with the initial dynamic equations and eliminating Lagrange multiplier with natural orthogonal complement technique lead to the comprehensive dynamic model. The variables of this model include the path of a reference point of the base and the position and orientation of the end-effector. The proposed approach will be applied on a car-like platform and a manipulator with 5 degrees-of freedom. The calculations for deriving such a model will be implemented by a program in Maple which can be used for control design and simulation purposes. The validity of the methodology is demonstrated using a second model and comparing the elements of these two models with each other. With trajectory generation for platform and manipulator generalized coordinates separately, set points for control system design will be provided. Motion generation for the platform, which due to the nonholonomic constraint has more sensitivity, will be dealt with by two motion modes. Inverting the model in terms of joint space variables, strict control of the work space variables is accomplished. Introducing state space variables and inverting the system into first order equations, the necessary preliminaries for control system design will be provided. Based on two simulation programs in Matlab, two controllers are designed with model-based algorithm (MBA) and Transposed Jacobian (TJ) control. Simulating different external conditions such as parameter perturbation, disturbances and noise, the robotic system behavior in the vicinity of real conditions will be examined. The results obtained show the merits of the TJ algorithm in controlling highly nonlinear and complex systems with multiple degrees- of freedom (DOF), without requiring a priori knowledge of plant dynamics, and with reduced computational burden which motivates further work on this algorithm

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Control of a class of uncertain nonlinear systems, which estimates unavailable state variables, is considered. A new approach for robust tracking control problem of satellite for large rotational maneuvers is presented in this paper. The features of this approach include a strong algorithm to estimate attitude, based on discrete extended Kalman filter combined with a continuous extended Kalman filter and attitude nonlinear model, and a robust controller based on sliding-mode with perturbation estimation. Estimation accuracy in this method is five times higher than other recent approaches based on Kalman filter. We have used sliding-mode controller in this paper. Not only the controller and the corresponding observer but also their composition must be robust. To make this controller robust against the uncertainty of parameters, the robust Kalman filter is used. Based on interval algebra, an upper bound and a lower bound are estimated for state variables of the system and considering these bounds in indicating the sliding conditions, stability of the controller in combination with the observer will be satisfied simultaneously. The simulation results show the capability of this method in spite of different uncertainty levels (up to %50).

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An important consideration in control issues is control of nonlinear systems. Sliding control is among those nonlinear controllers that can control the system desirably in the presence of unstructured uncertainties of carelessness in specifying parameters of the system. In sliding control, also called Variable Structure Control, the main objectives of the controller are achieved by introducing a sliding surface. One of the fundamental problems which may occur in sliding control is the chattering phenomenon on unwanted oscillation around the sliding surface. Different solutions are introduced to eliminate chattering. One of the commonest solutions is using a constant boundary layer round the sliding surface. In this paper, efforts are made to reduce chattering and to increase stability of the system by varying the sliding controller with a constant boundary layer. Finally, the mathematical model of a pendulum/cart in the presence of uncertainty is developed and the result of the simulation of the introduced controllers are compared.

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A physical model of gabion overflow dams was studied to determine the velocity profile and Reynolds shear stress. Physical tests were done under two different conditions of dam crest, overflow dams with impermeable and with permeable crests. Instantaneous velocity components over dam crest were measured by an ADV (Acoustic Doppler Velocimeter) instrument. This instrument is capable of measuring instantaneous velocity components with frequencies up to 25 Hz. Average velocity components and bed shear stress were extracted from ADV measurements. The results of this research show the effect of crest permeability on velocity and Reynolds shear stress. The magnitude of Reynolds shear stresses, horizontal velocity components, and absolute value of vertical velocity components under the permeable scenario are bigger than those of the impermeable scenario. Velocity distribution over the dam crest is different from the universal logarithmic profile.