This paper presents a novel optimal impedance voltage-controller for Electrically Driven Lower Limb Rehabilitation Robots (EDLR). To overcome the dynamical complexities, and handle the uncertainties, the proposed method employs an expected forward model of the actuator. The difference between this model’s output and the actual output represents the existing value of lamped uncertainty. A voltage-controller is designed based on this uncertainty estimator, which compensate for the uncertainties. Parameters of the controller have been optimized using genetic algorithms. Key contributions of this paper are I) estimation of the uncertainty by the expected model’s output, II) overcoming the changes in motor parameters, III) introducing a class of closed-loop system termed as “Repeatable”, and IV) designing an optimal impedance voltage-controller that is non-sensitive to the parameter variations. Significant merits of the approach are swift calculations, efficiency, robustness, and guaranteed stability. Furthermore, the simplicity of design, ease of implementation and model-free independent joint structure of the approach are noticeable. The method is compared with an adaptive robust sub-controller and a Taylor-series-based adaptive robust controller, through simulations in passive range of motion and active assistive rehabilitation exercises. The results show the superiority of the proposed method in tracking performance and the time of calculations.

In this paper, online controller scheme is proposed for voltage control of inverter-based distributed generations in islanded microgrid (MG). The MG has a completely nonlinear structure and its dynamics is constantly changing. As a result, linear controllers with constant and non-flexible coefficients are unable to provide a proper performance over a wide range of operating conditions. In response to this challenge, the present paper addresses an optimal online and nonlinear controller based on fuzzy logic. To improve the performance of the proposed controller, the differential evolution optimization algorithm is used to optimal tune its coefficient for the different MG operating scenarios. The goal of the proposed scheme is to control the voltage of the islanded MG at the standard level with the least transient state after the island's occurrence, as well as during load changes in island MG. In the proposed control scheme, proportional-resonant controller is used due to the advantages in order to voltage and current control of distributed generation units in the static frame space. The simulation results indicate the changing of the voltage controller coefficients, resulting in a better transient response, the elimination of the steady-state error and less harmonic distortion.

In this paper, a voltage-current mode controller is proposed to adjust both the input current and DC voltage of a boost-inverter in order to improve the performance of the photovoltaic (PV). The proposed system is based on fuzzy-making-decision predictive voltage-current controller (FPVCC). The simulation results show that in case of using the proposed FPVCC, the input current of a PV array is in the limited value (i.e., is approximate 27% of the short-circuit current of a PV panel) compared with a classical voltage controller, an over-current is passing through PV array (i.e., is about 155% of the short-circuit current). Also, the waveform of the output voltage is sinusoidal with low total harmonic distortion (THD), i.e. 4.21%, and the power obtained from a PV array with FPVCC is approximately 6 times of power obtained without a controller.

This paper develops a circuit model and controllers of fuel cell based distributed generation systems (DGS) in a standalone AC power supply. Dynamicmodel of the fuel cell is considered. To boost low output DC voltage of the fuel cell to high DC voltage and compensate for its slow response during the transient,two full-bridge DC to DC converters are adopted and their controllers are designed: a unidirectional fullbridge DC to DC boost converter for the fuel cell and a bidirectional full-bridge DC to DC buck/boost converter for the battery. For a three-phase DC to AC inverter, a discrete-time state space model in the stationary dq reference frame is derived and two discrete-time sliding mode controllers are designed: voltage controller in the outer loop and current controller in the inner loop. To demonstrate the proposed circuit model and control strategies, a simulation test-bed using Matlab/Simulink is developed and various results are given.

Three types of generator named wound rotor synchronous generator (WRSG), permanent magnet synchronous generator (PMSG) and doubly fed induction generator (DFIG) are usually used in variable speed horizontal axis wind turbines. Synchronous generator with controlled excitation voltage is a proper solution for power production in rotational speed lower than the nominal. In this paper, excitation voltage controller is designed to capture maximum energy of wind and pitch angle controller is designed to regulate the rotational speed of rotor when wind speed is over the rated value. Simulation results are verified by means of HIL (Hardware In the Loop) test setup. In this case, the function of controllers are examined in a turbulent wind condition on a simulator and the performance is analyzed. Finally, it is concluded that MPPT (Maximum Power Point Tracking) algorithm can be performed by controlling the excitation voltage of a synchronous generator in a below rated condition.

Considering the presence of different model parameters and controlling variables, as well as the nonlinear nature of DC to AC inverters; stabilizing the closed-loop system for grid current balancing is a challenging task. To cope with these issues, a novel sliding mode controller is proposed for the current balancing of local loads using grid-connected inverters in this paper. The closed-loop system includes two different controlling loops: a current controller which regulates the output current of grid-connected inverter and a voltage controller which is responsible for DC link voltage regulation. The main features of the proposed nonlinear controller are reactive power compensation, harmonic filtering and three-phase balancing of local nonlinear loads.  The developed controller is designed based on the state-space averaged modelling its stability and robustness are proved analytically using the Lyapunov stability theorem. The accuracy and effectiveness of proposed controlled approach are investigated through the PC-based simulations in MATLAB/Simulink.

Energy storage system (ESS) is generally used to manage the intermittency of the renewable energy sources (RESs). One of the important issues in microgrids is to supply high quality powers to consumers. This paper proposes the use of hybrid energy storages to improve the power quality under unbalanced load conditions for microgrids applications. Battery and SC are used as hybrid energy storages systems (HESSs) that provide low and high power frequency respectively. The proportional resonance(PR) and fuzzy controllers are, respectively, used to regulate the AC load voltage and DC bus voltage controller. The main advantages of the proposed energy management scheme are to reduce battery power fluctuations, better DC bus voltage regulation for generation and load disturbances, improvement system performance under unbalanced load conditions, reduced rate of charge/discharge of battery current, improved power quality feature under unbalanced load and transition conditions. The performance of the proposed method and control strategy is verified by using simulation studies in the MATLAB software environment.

With increasing the capacity and number of distributed energy resources (DERs), it is necessary to keep them connected to the microgrid during the disturbances. Two of the most important measures for satisfying this requirement are to provide the high quality voltage for feeding the critical loads and to limit the electronically-coupled DERs current and voltage. This paper investigates the performance of hierarchical control structure of islanded microgrids during and after the fault conditions. The primary control level of this control structure is equipped with the hybrid reference frame limiting strategy to limit the voltage-sourced converter (VSC) current and voltage. The effect of employing the concept of independent control of natural reference frame in both primary and secondary control levels and the influence of using the conditional integration method in the voltage controller of secondary control level on the microgrid performance during and after fault condition are investigated. The study results are demonstrated through several time-domain simulations of symmetrical and asymmetrical faults.

Robot manipulator with flexible-joints is very complex nonlinear system whose control is one of the most challenging issues in the control word. Therefore, the design of voltage-based controller in addition, using of order reduction methods can reduce the complexity of the control law in such systems. This paper proposes a voltage controller for flexible-joint robot manipulators based on singular perturbation method. The presented control approach has all three advantages of the singular perturbation method for model order reduction, the proper structure of Port-Hamiltonian systems, and voltage control strategy (VCS). In this approach, the robot manipulator model is divided into three sub-systems, slow, medium and fast sub-systems. Each of the sub-systems is controlled using separate controller. In addition, the stability of these sub-systems and ultimately the whole system are proved. Unlike other related works, in this work the tracking error system is considered from the beginning, and by singular perturbation method, a controller is designed to stabilize the tracking system. Moreover, in the suggested voltage-based controller unlike torque control strategy, the electrical model of actuators is used. The main advantages of proposed approach are simple structure, using only velocity of motors, the position of the joints as a control signal and considering the electrical model of the actuators. So, practical implementation of this controller will be with much less effort, compared to the methods like feedback linearization or other controllers in related works. Moreover, using the Lyapunov-based method, the ultimate bounded stability of the closed loop system is proved. Then, some simulations are provided for tracking, regulation, robustness and response speed purposes. Since, the controllers for every sub-system are designed separately, also, the control signal parameters such as joints position, motors velocity, and motors current can be simply measured, therefore, the structure of designed controller is very simple and practically implementable. As the simulation results confirm, the performance of this controller is appropriate, even when external disturbances are present or the frequency of reference signal increases. At last, by comparison and analysis of the simulation results between the presented approach and a related work, the suitable tracking performance of the suggested controller is shown.

This paper proposes the improved hierarchical- based control of Microgrid based on proportional and multi-resonance controllers to compensate for harmonic distortion of nonlinear loads. Moreover, the probable transition of MG, especially from grid-connected to unplanned islanding and unintentional MG resources outage are studied. In current and voltage controllers of three-phase VSIs which are located in the inner level, the proportional and multi-resonant controllers are implemented. To attain proper decoupled (P-Q) power-sharing, a selective harmonic type virtual impedance, and a droop-based control are implemented at the primary level. Next, to reach better restoration and subsequently, seamless transition in accidental islanding, unintentional MG-DG’s outage, and synchronization process, the advanced three-phase SRF-PLL with in-loop MAF along with a simple adaptive lookup table are implemented in the secondary level of control. The MATLAB/Simulink simulation results verified that the proposed method improved the performance of control, effectiveness, and robustness in upstream or local grid variation.