Sharif Digital Repository

Challenging problems in the design and analysis of photovoltaic (PV) systems stem from the nonlinear current–voltage (I–V) characteristics of solar cells. This paper presents an analytical analysis based on a describing function method to investigate the transient and steady-state characteristics of a three-phase single-stage grid-connected PV system. In this study, the nonlinear I–V characteristic of the PV array is linearized around the operating point. The nonlinear dynamic of the maximum power point tracking controller is divided into two parts of continuous and discrete. For the continuous part, the common small-signal linearization is applied, while for the discontinuous part, a... 

This study describes an adaptive sliding mode technique for attitude and position control of a rigid body insect-like flapping wing model in the presence of uncertainties. For this purpose, a six-degrees-of-freedom nonlinear and time-varying dynamic model of a typical hummingbird is considered for simulation studies. Based on the quasi-steady assumptions, three major aerodynamic loads including delayed stall, rotational lift and added mass are presented and analyzed, respectively. Using the averaging theory, a time-varying system is then transformed into the time-invariant system to design the adaptive controller. The controller is designed so that the closed-loop system will follow any... 

A theoretical model was developed to describe a dynamic process involving an enzymatic reaction and diffusion of reactants and product inside glucose sensitive composite membrane. The composite membrane consisted of nanoparticles of a weakly acidic polymer, glucose oxidase and catalase embedded in a hydrophobic polymer. Time- and position-dependent diffusivity of involved species was considered in the model. Donnan equilibrium was used to find concentrations of buffer ions inside the membrane. The profiles of pH, species concentrations, volume fraction of swollen gel, polymer and water-filled space, as well as solute diffusivity inside the membrane were predicted by the model as a function... 

Inverse dynamics analysis as well as the generation of an optimal goal oriented human motion both lead to the problem of finding suitable activations of the redundant muscles involved. This paper employs an iterative feedback tuning approach to perform the forward dynamics simulation of the human musculoskeletal system during level walking. A modified form of the proportional-integral-derivative (PID) controller is proposed to stabilize the movement and provide tracking of problems of the desired lower extremity joint profiles. Controller parameters were determined iteratively using an optimization algorithm to minimize tracking errors during forward dynamics simulation. Static optimization... 

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... 

DC-link voltage and output current control loops are two cascaded loops in the control structure of grid-connected inverters. A high DC-link voltage loop bandwidth (DCL-BW) provides more disturbance rejection capability for the control loop and is preferred from control system perspective. However, for stability issues, this BW is limited and must be sufficiently less than that of the current control loop. Among the different control schemes, instantaneous active reactive control (IARC) method provides the highest possible DCL-BW (i.e., 0.02 × switching frequency). Having this degree of freedom in the controller design, a proper methodology should be defined for selection of DCL-BW. In this... 

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... 

This paper presents a closed-loop laser cladding process used in nonplanar deposition of desired metallic materials. In the proposed system, the deposited layer geometry is continuously controlled via a sliding mode controller (SMC). The controller, which uses the scanning speed as the control input, is designed based on a parametric Hammerstein model. The model is a parametric dynamic model with several unknown parameters, which are identified experimentally using the recursive least squares method. The designed SMC is robust to all model parameters' uncertainties and disturbances. The results showed that the tracking accuracy improves and the chattering effect reduces if an integrator on... 

Two-degree-of-freedom (2-DOF) electrical machines require position sensors for their motion control. In comparison with using two independent sensors, using a 2-DOF sensor enhances the closed-loop control system's performance. However, due to the 3-D structure of the 2-DOF sensor, its performance evaluation needs 3-D analysis. Also, due to helical motion the accuracy deterioration of the sensor, under mechanical faults needs more attention. Although the finite element method (FEM) is the best way to simulate such sensors, most of the commercial packages for transient finite element simulations are not able to consider two separate motions simultaneously. Furthermore, FEM has a high... 

Background: Comparison of the kinematic variability and dynamic stability of the trunk between healthy and low back pain patient groups can contribute to gaining valuable information about the movement patterns and neuromotor strategies involved in various movement tasks. Methods: Fourteen chronic low back pain patients with mild symptoms and twelve healthy male volunteers performed repeated trunk flexion-extension movements in the sagittal plane at three different speeds: 20 cycles/min, self-selected, and 40 cycles/min. Mean standard deviations, coefficient of variation and variance ratio as variability measures; maximum finite-time Lyapunov exponents and maximum Floquet multipliers as... 

Single-phase ac-dc Power Factor Correction (PFC) rectifiers have attracted considerable attention in recent years due to the adoption of increasingly stringent power quality regulations. In this paper a nonlinear model predictive controller design for a single-phase Flyback PFC rectifier is presented. After approximation of the tracking error in the receding horizon by its Taylor-series expansion to a specified order, an analytic solution to the model predictive control (MPC) is developed and a closed-loop nonlinear predictive controller is introduced. The main advantage of this digital control method is that unity power factor can be achieved over wide input voltage, load current range and... 

The aim of this study is to employ feedback control loops to provide a stable forward dynamics simulation of human movement under repeated position constraint conditions in the environment, particularly during stair climbing. A ten-degrees-of-freedom skeletal model containing 18 Hill-type musculotendon actuators per leg was employed to simulate the model in the sagittal plane. The postural tracking and obstacle avoidance were provided by the proportional-integral-derivative controller according to the modulation of the time rate change of the joint kinematics. The stability of the model was maintained by controlling the velocity of the body's centre of mass according to the desired centre of...