Cascaded Control Loops Research Articles

Hybrid Model Predictive Current and Voltage Control for LCL-Filtered Grid-Connected Inverter

The inductive-capacitive-inductive (LCL)-filtered grid-connected inverter features a high-order plant, complex parameter design, and vulnerability under disturbance factors. The transfer-function-based active damping (TFAD) scheme is commonly adopted to mitigate the resonance problem but with cascaded control loops and high design complexity. On the contrary, multiobjective optimization can be easily realized in model predictive control (MPC) via a predefined cost function. As the coupling dynamic among state variables, a hybrid cost function, which contains the grid current, inverter-side current, and capacitor voltage tracking, is designed to guarantee system stability. The optimized control law is generated according to the minimization of the designed cost function, which is further transformed into the driving pulse by the modulator stage. In this process, the weighting factors are exactly arranged to place the desired closed-loop poles. The control law can ensure zero error tracking of grid current reference, which is better than the state feedback way. The hybrid cost function with modified reference is also proposed to reject the harmonics caused by the unhealthy grid condition. The control delay is compensated by using the forecast reference and the predictive state variable estimated from the observer. The proposed scheme performs the intuitive concept, reduced tuning complexity, and fast dynamic response. Simulated results and experiments are presented to verify the proposed control strategy.

Modeling and control of DC/AC converters for photovoltaic grid-tie micro-inverter application

• Modeling and control for a Photovoltaic-based microinverter system. • An efficient maximum power point tracking algorithm is implemented here. • Both state-space modeling and small-signal analysis are carried out. • Enhancement of transient and dynamic performance by using a cascaded controller. • Finally, a 500 Watts, 110 V, 50 Hz microinverter prototype is fabricated and tested. This paper is devoted to the modelling and control for a low cost, high-power quality single-phase voltage source inverter (VSI) for a grid-tied PV-based micro-inverter system. The first stage includes a high-efficiency isolated boost dual half-bridge dc-dc converter topology which interfaces to the PV panel and produces a dc-link voltage. The second stage is comprised of a single-phase VSI circuit. A state-space average model for a 1-Φ VSI with low-pass LCL filter topology has been developed. The steady-state and dynamic performance of the system can be improved by using three cascaded control loops. The fast inner current controller is adopted to regulate the inductor current of the inverter thereby improved dynamic response of the system. The small-signal analysis is conducted to assess the robustness of the proposed controller. An outer dc bus voltage control loop is employed to keep the constant voltage across the dc-bus capacitor. Next, the grid-side current loop is employed to regulate the grid current with a low total harmonic distortion level. The Bode-diagrams are plotted to assess the stability of each control loops. Finally, a 500 Watts, 110 V, 50 Hz micro-inverter prototype is fabricated and tested. The simulation and experimental results are given to show the effectiveness of the proposed control scheme.

Control and Operation of a Hybrid Actuator for Maglev Applications

Abstract The hybrid actuator presented in this article is meant to enable stationary and slow dynamic levitation in Maglev applications. The term ‘hybrid’ refers to the design of the actuator, which is a combination of permanent magnets (PM) and electromagnets. This paper presents an analytically computable control algorithm for the said hybrid actuator. The theory of magnetic circuits is summarized shortly and used to derive a cascaded control loop consisting of an inner current controller and an outer air gap controller. Since the uncontrolled hybrid actuator is inherently unstable, the system has to be stabilized. By introducing a PID-controller into the air gap control loop, the unstable behaviour of the uncontrolled system is changed into the system behaviour of a damped harmonic oscillator. The advantage of this approach is that the computed controller parameters of the PID-controller can easily be adjusted, so the system behaviour of damping and eigenfrequency can be selected within a certain range. For the execution of the control algorithm, a microcontroller (MCU) is used and for precise air gap measurement, an eddy current sensor is installed. Finally, the behaviour of the current- and air gap controller is discussed for different measurement results and the adjustable system behaviour of the damped harmonic oscillator is presented.

A Full State-Variable Direct Predictive Control for Islanded Microgrids With Parallel Converters

In this work, we propose a high-quality control solution for islanded microgrids with multiparallel power converters; it uses a full state-variable direct model predictive control (FSV-DMPC) and has a simple structure. Unlike the conventional cascaded control loops, the proposed FSV-DMPC solution tracks the optimal reference generated by a robust droop loop using a unified cost function. This proposal enables the FSV-DMPC to be inserted into the entire control framework with plug-and-play capability; it is robust to parameter variations while also guaranteeing dynamics and stability. We conduct a deep analysis of the proposed approach, taking into account both the characteristics of the solution and the bounded stability of the system. Through comprehensive comparative studies with a classical double-loop linear controller, we validate that our solution achieves superior output voltage regulation during the load transients in terms of voltage error and settling time. Meanwhile, similar steady-state performances are accomplished for both methods. Finally, we verify our approach experimentally in different scenarios through a lab-constructed microgrid test bench. Experimental data confirm that the proposed approach achieves excellent steady-state and transient performances and obtains accurate load sharing.

Extended state observer-based robust control of an omnidirectional quadrotor with tiltable rotors

A quadrotor with tiltable rotors is a kind of omnidirectional multirotor aerial vehicle (MAV) that has demonstrated advantages of decoupling control of position from the control of orientation. However, quadrotors with tiltable rotors usually suffer from Coriolis term, modeling error and external disturbance. To this end, the extended state observer (ESO)-based controller is designed to estimate and compensate for the above adverse effects. Especially, the controller involves position and attitude controller in parallel. The attitude controller is made up of cascade control-loops: an outer quaternion-based attitude control-loop and an inner ESO-based Proportional derivative angular velocity control-loop. Similarly, the position controller consists of an outer proportional position control-loop and an inner ESO-based PD velocity control-loop. Besides, a linear control allocation strategy, which allocates the controller outputs to tilting angles and motor speed directly, is proposed to avoid the nonlinear allocation matrix. Extensive simulations and flight tests are carried out to illustrate the effectiveness and robustness of the proposed ESO-based controller.

Impedance Modeling and Stability Analysis of VSG Controlled Grid-Connected Converters with Cascaded Inner Control Loop

This paper develops the impedance models of grid-connected converters under the virtual synchronous generator (VSG) strategy with a cascaded inner control loop and analyzes the system stability of VSG controlled converters with different kinds of weak grid. Different from existing small-signal models with high dimensions, a single-in-single-out (SISO) impedance model with simple mathematical expression is obtained in this paper, which is applied to identify the influence of the cascaded control loop on impedance characteristics and system stability. It is found that the impedance characteristics of VSG controlled converters can become capacitive below the fundamental frequency, and it is mainly caused by the voltage controller in the cascaded control loop of the VSG strategy. Impedance-based stability analysis shows that the capacitive impedance characteristics can benefit the compatibility of converters operated with the series-compensated weak grid, but may deteriorate the system stability with the inductive weak grid, which can be avoided by increasing the proportional coefficients of the cascaded voltage and current controllers or applying a larger virtual resistor to reduce the negative resistance in the capacitive frequency range. Experiments based on the control-hardware-in-loop (CHIL) platform were carried out to verify the developed analytical models and possible system instable cases.

Control strategies for seamless transfer between the grid-connected and islanded modes of a microgrid system

Design of control strategies for Distributed generation systems is very important to achieve smoother transition between the grid connected and islanding modes of operation. The transition between these two modes of operation should be seamless, without any severe transients during the changeover. In this paper, two different control strategies namely inverter output current control and indirect grid current control for the seamless transfer between the modes of operation has been explored for the suitability. The design and analysis of the cascaded control loops based on Proportional Integral (PI) controller has been dealt in detail for both inverter output current control and indirect grid current control strategy. Control parameters are designed using the control system toolbox in MATLAB. A 10kW grid connected microgrid system has been designed and simulated in MATLAB/Simulink and the results are presented under grid connected operation, islanding operation and the transition between the modes considering fault condition in the grid side. The simulation studies are carried out using both the control strategies and the results are presented to validate the design methodology.

Feedback controller for restoring the basal hemodynamic condition with a rotary blood pump used as left ventricular assist device

This work deals with the use of pulsatility indices in designing a ventricular assist device (VAD) control system. The control objective is to restore the patient's basal hemodynamic energy. The proposed control system structure comprises a hierarchy of cascaded control loops. At the highest level, there is the pressure feedback control loop, and at the lowest level, there are feedback control loops for both the rotational speed and the armature current. The reference signal for the high-level controller has a pulsatile profile. The generation of this pulsatile profile employs a procedure that takes into account the fulfillment of physiological specifications. Among the considered physiological requirements, there is the mean arterial pressure (MAP), the energy equivalent pressure (EEP), or the surplus hemodynamic energy (SHE). The gains of the low-level controllers are calculated based on the time-domain specification and the VAD mathematical model parameters. On the other hand, high-level controller gains are determined by using integral performance error criteria as tuning rule. The results obtained in silico indicate the feasibility of the proposed control system as well as its related design procedure. With the generated reference pump profile, it is possible to restore the basal hemodynamic condition for a heart failure patient. Besides, the restored basal hemodynamic condition is quite robust since it is maintained even under variations of the systemic resistance and heartbeat rate.

Development of an adaptive control strategy for the three‐phase grid side converter with wide range of parametric and load uncertainties

A new approach is proposed here to control the two-level three-phase grid side converter. The proposed control strategy is an extended state observer (ESO)-based adaptive control, consists of two cascaded control loops. The adaptive-multi-input multi-output controller is employed in the inner control loop, which is used to regulate the active and reactive component of the grid currents. The outer-loop consists of an adaptive controller augmented with an ESO to regulate the DC bus voltage. The goal of the proposed control strategy is to maintain a unity power factor while keeping the DC bus voltage constant under a sudden change in load demand at DC side, i.e. change in the resistive load connected across the DC bus. The proposed adaptive controller in the inner control loop does not require any information about the parameters associated with the grid filter (L-filter), and hence, it is found to be more robust towards parametric variations. The ESO-based adaptive controller is compared with the classical proportional–integral control in real-time, and the comparison indicates that the proposed controller not only accomplishes a nearly flawless tracking performance but also delivers a complete robust performance against resistive load variation across the DC bus.

Robust Model Predictive Speed Control of Induction Motors Using a Constrained Disturbance Observer

This paper proposes an offset-free speed control method for induction motors using a model predictive control and disturbance observer. The proposed method has cascade control loops i.e. inner-loop model predictive torque control and outer-loop robust speed control. The outer-loop robust speed controller is composed of a stabilizing MPC and a discrete-time disturbance observer. The disturbance observer estimates the effects of the parameter uncertainties and the load torque to yield an offset free speed tracking, while taking the torque limit of the motor into account. This speed controller generates the reference torque for the inner-loop torque controller. A loss-minimizing continuous control set model predictive torque controller(CCS-MPTC) considering the current and voltage constraints of the motor is used as the inner-loop torque controller. Experimental results show a significant performance improvement when the torque constraints are considered in the DOB design.

A Novel DC Fault Ride Through Control Methodology for Hybrid Modular Multilevel Converters in HVDC Systems

Modular Multilevel Converter (MMC) is an established technology for HVDC or Multi-Terminal DC (MTDC) systems, due to its advantages over classical Voltage Source Converters (VSCs) such as two or three level VSCs. To achieve a full control of all state variables, it is essential to implement energy-based method in which a cascade control loop is employed to regulate all state variables including ac and differential currents, and stored energy within MMC arms. In addition to normal operation control, dc Fault Ride Through (DC-FRT) capability of the MMC is a crucial and challenging control issue especially for overhead line HVDC system where non-permanent dc fault occurrence is statistically more probable. Furthermore, the main problematic technical obstacle to develop HVDC/MTDC grids is the lack of mature dc fault protection. Since conventional control in normal operation cannot be employed in case of dc fault, an efficient control strategy is indispensable. The principal objectives of the novel control methodology are to (i) obtain DC-FRT capability, (ii) decay short circuit current to zero, (iii) secure the MMC through leg and arm energy balancing, (iv) support ac grid as a Static Synchronous Compensator (STATCOM) and (v) resume normal operation after dc fault clearance. The simulation results verify the validity of proposed control strategy to fulfill the abovementioned objectives in dc fault operation of the hybrid MMC.

OPTIMIZATION OF PID PARAMETERS BASED ON PARTICLE SWARM OPTIMIZATION FOR BALL AND BEAM SYSTEM

This paper introduces the application of an optimization technique, known as Particle Swarm Optimization (PSO) algorithm to the problem of tuning the Proportional-Integral-Derivative (PID) controller for a linearized ball and beam control system. After describing the basic principles of the Particle Swarm Optimization, the proposed method concentrates on finding the optimal solution of PID controller in the cascade control loop of the Ball and Beam Control System. Ball and Beam control system tends to balance a ball on a particular position on the beam as defined by the user. The efficiency of Particle Swarm Optimization algorithm for tuning the controller will be compared with a classical method, Trial and Error method. The comparison is based on the time response performance. The two tuning methods have been developed by simulation study using Matlab\ m-file software. The evaluations show that Evolutionary method Particle Swarm Optimization (PSO) algorithm gives a much better response than trial and error method.

Uncertainty and Disturbance Estimator-Based Global Trajectory Tracking Control for a Quadrotor

This article presents an uncertainty and disturbance estimator (UDE) based global trajectory tracking control strategy for a quadrotor. The main contribution of this article is the fusion of the UDE technique with the unit quaternion to achieve the robust global full degrees of freedom trajectory tracking control with experimental demonstrations. The novelty of this article lies in the development of the UDE-based global tracking technique to the overall system (attitude and position) with a quaternion-based nonlinear reference model. The UDE-based attitude and position controllers are derived from the unit quaternion-based quadrotor dynamics with a cascade control structure to deal with underactuation, model uncertainties, and external disturbances. The attitude controllers that are developed with the backstepping techniques avoid the rotation matrix calculation and the unwinding problem. The position controllers are derived using the thrust-vectoring approach. A nonlinear unit quaternion-based reference model is developed to achieve the time-scale separation for the cascade control loops. The stability analysis of the closed-loop system is conducted with the Lyapunov method. Extensive flight experiments are conducted to demonstrate the global singularity-free tracking property and the superior robustness of the proposed controller.

Nonlinear guidance law algorithm applied to a small unmanned surface vehicle

The unmanned surface vehicles have been used most frequently in recent years in different applications, like environmental research. For this vehicles to accomplish their autonomous missions, a path-following algorithm is necessary to reduce the cross-track error in the presence of environmental disturbance. This article presents a control scheme based on the path-following nonlinear guidance law for a small unmanned surface vehicle called Krick Felix which follows a straight path. A dynamic model of 3 degrees of freedom for this vehicle is presented. The control scheme consists of a cascade control loop that is capable of guaranteeing zero cross-track error in the presence of environmental disturbance without adding an integral action. A nonlinear Lyapunov stability analysis is carried out for this control scheme taking in consideration the dynamics of both the inner loop and the external loop. The simulation was realized by implementing the 3-degree-of-freedom nonlinear model of the Krick Felix. The simulation also took account of the environmental factors, that is, marine currents. An experimental test is carried out with the Krick Felix where the control scheme present satisfactory results.

A Universal Controller Under Different Operating States for Parallel Inverters With Seamless Transfer Capability

A universal controller is proposed in this article to operate parallel inverters in both grid-connected (GC) state and standalone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The universal controller is mainly composed of frequency-locked-loop and three cascaded control loops: a grid current loop, capacitor voltage loop, and inductor current loop. A proportional-integral regulator is adopted in the grid current loop, and a limiter is inserted after the integrator. In the GC state, the proposed controller accurately regulates the grid current of an individual inverter. When islanding occurs, the proposed controller can automatically convert from grid current control to v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> -i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> -based droop control; critical islanding detection is not needed. This universal controller has several advantages. First, it can realize seamless transfer of parallel inverters; microgrid reliability can be guaranteed in the SA state, and power sharing can be achieved among parallel inverters without communication lines. Second, compared with droop control, it can regulate the grid current accurately and output constant power when the grid voltage fluctuates in the GC state. Third, the grid current harmonics in the GC state and capacitor voltage harmonics in the SA state can be mitigated. Simulation and experimental results verify the effectiveness of the controller.

Non-Linear Tank Level Control for Industrial Applications

Tank level control is ubiquitous in industry. The focus of this paper is on accurate liquid level control in single tank systems which can be actuated continuously and modulation of the level setpoint is also required, for example in cascade control loops or supervisory Model Predictive Control (MPC) applications. To avoid common problems encountered when using fixed gain or adaptive/gain scheduled schemes, an accurate technique based around feedback linearization and Proportional Integral (PI) control is introduced. This simple controller can maintain linear performance over the full operating range of a uniform tank. As will be demonstrated, the implementation overhead compared to a regular PI controller is negligible, making it ideal for industrial implementation. Implementation details and parameter identification for adaptive implementation are discussed. Simulations coupled with experimental results using a large-scale laboratory level control system using commercial industrial control equipment validate the approach, and illustrate its effectiveness for both level tracking and disturbance rejection.

A Self-Adaptive Controller for Inverter With Seamless Transfer and Automatic Pre-Synchronization Capability

A self-adaptive controller is proposed to operate inverter in both grid-connected (GC) state and stand-alone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The controller is mainly composed of phase-locked-loop (PLL) and two cascaded control loops. The role of PLL is not only to estimate the frequency but also to generate the reference of voltage loop. When the grid is normal, the reference and the feedback value of voltage loop are always equal and the proposed controller accurately regulates the grid current of inverter. When islanding occurs, the inverter can automatically convert from current control mode to voltage control mode without critical islanding detection. The self-adaptive controller has several advantages. First, compared with direct current control, it can operate in both strong and weak grid conditions. Second, its dynamic performance in the GC state is much better than that of indirect current control. Third, when the grid returns to normal, the pre-synchronization of inverter's output voltage and grid voltage can be achieved automatically without any extra control algorithm. Finally, simulation and experimental results are provided to verify the effectiveness of proposed self-adaptive controller.

An empirical Study Investigation of Task Allocation Process Barriers in the Context of Offshore Software Development Outsourcing: An Organization Size Based Analysis

This research presented a holistic approach in determining the trade-off optimized Proportional-Integral-Derivative (PID) tunings for both servo and regulatory controls of the cascade control loop by using Genetic Algorithm (GA). Performance of GA-based PID tunings was significantly compared with the IMC-based single loop tunings and conventional cascade control tunings. GA-based PID tunings eliminated the complicated mathematic calculations in obtaining the correlation PID tuning values and also reduce the dependency on engineering knowledge, experience, and skills. The performance of transient and steady-state responses was compared through time domain specification, performance index, and process response curve. It is concluded that the GA-based PID tunings for the cascade control loop had produced the best result for both servo and regulatory control objectives, which is eventually determined.

Full Converter Control for Variable-Speed Wind Turbines Without Integral Controller or PLL

This article addresses a controller for a full converter of variable-speed wind generators composed of permanent magnet synchronous generators. Conventionally, several proportional–integral (PI) controllers and a phase-locked loop (PLL) are used in the full converter for voltage-oriented control. The use of cascaded control loops with PI controllers degrades the control performance due to the windup of the integrators and complicates the tuning task. In this article, we propose a new full converter control scheme based on predictive control and a single P-controller in the $\alpha \beta$ stationary frame to replace the integrators and the PLL. The proposed controller can reduce the parameter tuning effort, is free from integrator windup, and is robust to symmetrical and asymmetrical voltage dips. The effectiveness of the proposed method is verified through simulations.

Practical Implementation of the Backstepping Sliding Mode Controller MPPT for a PV-Storage Application

This study presents a design and an implementation of a robust Maximum Power Point Tracking (MPPT) for a stand-alone photovoltaic (PV) system with battery storage. A new control scheme is applied for the boost converter based on the combination of the adaptive perturb and observe fuzzy logic controller (P&amp;O-FLC) MPPT technique and the backstepping sliding mode control (BS-SMC) approach. The MPPT controller design was used to accurately track the PV operating point to its maximum power point (MPP) under changing climatic conditions. The presented MPPT based on the P&amp;O-FLC technique generates the reference PV voltage and then a cascade control loop type, based on the BS-SMC approach is used. The aims of this approach are applied to regulate the inductor current and then the PV voltage to its reference values. In order to reduce system costs and complexity, a high gain observer (HGO) was designed, based on the model of the PV system, to estimate online the real value of the boost converter’s inductor current. The performance and the robustness of the BS-SMC approach are evaluated using a comparative simulation with a conventional proportional integral (PI) controller implemented in the MATLAB/Simulink environment. The obtained results demonstrate that the proposed approach not only provides a near-perfect tracking performance (dynamic response, overshoot, steady-state error), but also offers greater robustness and stability than the conventional PI controller. Experimental results fitted with dSPACE software reveal that the PV module could reach the MPP and achieve the performance and robustness of the designed BS-SMC MPPT controller.