Multi-loop Control Research Articles

Digitally Implemented Charge Control for LLC Resonant Converters

This paper proposes a digitally implemented charge control for LLC resonant converters. By analyzing the small-signal characteristics of the resonant capacitor, it is revealed that the resonant capacitor has exactly the same functionality as a resettable integrator in the charge control for a PWM converter. Based on the analyzed results, the proposed charge control is implemented simply by sensing the voltage across the resonant capacitor in a switching period. The proposed method features simple implementation with resistive voltage divider and suitability for digital control as well as good performance as other multiloop controls have. The design guideline for the charge controller is given based on the loop gain analysis. Finally, the operation and performance of the proposed method are verified with a 240-W LLC resonant converter.

Design of multiloop PI controllers based on quadratic optimal approach

The LQR methodology is extended to produce multiloop PI controllers in this study. With this method, a multivariable P controller is first obtained and then diagonalized using an iterative procedure. The resulting controller is further complemented with the integral action in a similar process. The proposed tuning process explores different values of the control weight matrix to balance the output error and control signal in both stages, and is refined through additional indices related with the error to reference tracking, disturbance rejection and the associated control use. In addition, the diagonalization procedure is generalized to obtain multiloop versions of existing multivariable PI controllers. The developed theory was applied to the design of multiloop controllers for a distillation column as well as the diagonalization of existing controllers for a nonlinear CSTR. The tuning procedure allowed the synthesis of multiloop PI controllers with performance indices comparable to those reported by other authors. Furthermore, diagonalized controllers exhibited a similar operation as the original multivariable ones.

Distributed FACTS stabilization scheme for efficient utilization of distributed wind energy systems

Summary Because of the growing usage of wind energy systems worldwide, more challenges in power quality and microgrid stabilization domains have emerged. In this paper, a new distributed FACTS-based green plug-switched filter capacitor (GP-SFC) filter is presented for efficient utilization of microgrid-connected wind energy systems. The primary goals are improving the energy use, stabilizing voltage dynamically, enhancing power quality, and improving the power factor at the common interface AC buses of the microgrid-connected wind system. The proposed GP-SFC topology is modelled and simulated using MATLAB/Simulink software. A dynamic multiloop error-driven controller is proposed for adjusting the pulse switching pattern of the variable speed wind turbine system. In addition to the low cost of the proposed GP-SFC filter, the results show that it can promote the use of wind energy system under load changes and temporary fault conditions. Besides, the proposed GP-SFC guarantees high-speed controllability while maintaining its capability for power factor correction and voltage support using its switched shunt capacitor. Also, the system performance with the proposed filter is compared with the system with a conventional D-STATCOM compensator. The results validate the effectiveness and robustness of the GP-SFC topology.

Multi-loop nonlinear control design for performance improvement of LTI systems

This paper puts forward a multi-loop nonlinear control (MLNC) strategy to overcome the limited performance of LTI controllers due to the so-called “waterbed” effect. According to “Bode's sensitivity integral”, increasing the bandwidth or additional integral gain of LTI controller to improve the low-frequency disturbance attenuation irrefutably increases the sensitivity to high-frequency disturbances or measurement noise. Hence, it is impossible to attain the best of both worlds in the case of linear controllers. Therefore, with an aim to improve the transient and steady state performance of linear controllers, in this paper, a nonlinear control framework using circle criterion method and saturation nonlinearity, which adjusts the integral gain based on the error threshold, is discussed. The global asymptotic stability (GAS) of the MLNC strategy is theoretically proved using LaSalle's invariance principle and experimentally validated using measured frequency response function (FRF). Moreover, the performance of the MLNC strategy is compared with that of the multi-loop linear control (MLLC) strategy on a benchmark magnetic levitation system for tracking application. The cumulative power spectral density (CPSD) of tracking error, which is used as the performance index to assess the overall closed loop performance, accentuates that MLNC can yield better steady state and transient performance compared to MLLC scheme.

Multiloop Integral Controllability Analysis for Nonlinear Multiple-Input Single-Output Processes

Multi-loop integral control is still one of the most popular control strategies in industry due to its simplicity, efficiency, offset free tracking, and capability for fault tolerance. Skogestad and Morari introduced Decentralized Integral Controllability (DIC) to investigate the decentralized unconditional stability under multi-loop integral control for square systems. However, in engineering practice, some multivariable processes may not be square, which often utilize multiple redundant control inputs for the regulation of only one single output. This study extends the concept of Decentralized Integral Controllability to non-square systems, and presents sufficient conditions for Multiple-Input Single-Output nonlinear processes based on singular perturbation analysis. The proposed controllability analysis method is applied in the control of a real time temperature control system and achieves desired temperature tracking results.

Robust H∞ switching MIMO control for a plasma time-varying parameter model with a variable structure in a tokamak

The paper deals with design and simulation of hierarchical multi-loops feedback magnetic plasma control system on the base of experimental data for the operating Globus-M spherical tokamak in line with the special methodology developed. The components of the system are designed to achieve high-performance operation during short pulses of plasma discharges about 200 ms. The plasma equilibrium reconstruction algorithm with regularization in terms of moving filaments without iterations was developed. The LPV plant model was derived from reconstructed plasma equilibriums as a distributed parameter system with time-varying parameters. A switching plasma shape multivariable controller was designed by the H∞ robust loop-shaping approach for magnetic field at X-point and poloidal fluxes on plasma separatrix (isoflux control). The novelty of the plasma shape control system is the application of robust switching control with a controller state vector matching. The feedback control was applied to the whole discharge with the transition from a limiter phase to a diverter phase when the plasma changed its magnetic topology as a controlled structure.

Backstepping Control of Photovoltaic-Grid Hybrid Power Feed Water Pump

We are considering the problem a controlling of photovoltaic-grid pumping system. The system consists of a PV panel, a PWM AC/DC rectifier connected to a single-phase grid, an association of a PWM DC/AC inverter - squirrel cage induction motor, and a centrifugal water pump. The control objectives are fourfold: (i) forcing the pump water flow rate to track any reference signal (ii) regulating the rotor flux norm to its nominal value (iii) regulating the DC Link voltage to a reference value to meet the maximum power point tracking (MPPT), (iv) and the power factor correction (PFC). To meet these control objectives, we first develop a nonlinear model of the whole controlled system in the Park-coordinates. Then, a multi-loop nonlinear controller is synthesized using the backstepping design technique. In addition to closed-loop global asymptotic stability, it is proved that all control objectives (water flow rate tracking, rotor flux regulation, DC link voltage regulation and unitary power factor) are asymptotically achieved.

Lyapunov Based 3D Path Following Kinematic Controller for a Fixed Wing UAV

Path following is a basic autonomous navigation capability for UAV’s and in this paper we present a nonlinear 3D path following kinematic controller. In order to solve this problem, we use the typical multi-loop control structure in which the navigation controller directs the aircraft to the desired path, while the inner loop acts as an autopilot which stabilizes the nonlinear dynamics and tracks the command generated by the outer loop. Under the assumption that the autopilot is well tuned and the airspeed, fight-path angle, and bank angle states converge to the desired response to their commanded values, we use a kinematic model as a suitable control model for the fixed wing UAV. With the introduction of a virtual target, the proposed Lyapunov based kinematic controller for outer loop can steer the UAV asymptotically to approach the desired path. The proposed controller is validated in a 6-DoF simulation environment platform named MAV3DSim.

Nonlinear Control of PV System Connected to Single Phase Grid through Half Bridge Power Inverter

This paper addresses a modeling and nonlinear control of photovoltaic systems which includes photovoltaic panels, a half bridge inverter with capacitive divider and an LCL filter connected to single-phase grid. We seek the achievement of three control objectives: i) ensure that the PV panels generate its maximum power ii) regulating the DC link voltage to track its reference provided by MPPT block, iii) achieve a satisfactory power factor correction (PFC) with respect to the power supply net. To meet these objectives a multi-loop controller is designed based on nonlinear approach applied to the 5th order nonlinear model. The simulation results have been performed through MATLAB/SimPowerSystems environment and show that the designed controller meets its objectives.

Spectral element formulation for an electrostrictive material embedded in a beam

In this paper, a spectrally formulated finite element in frequency domain for an electrostrictive material embedded in a composite beam is derived. The presence of quadratic non linearity of the electric field in the constitutive equation poses some challenge in the formulation of the model and we use convolution in the frequency domain. The formulation is carried out using Hamilton’s theorem and this is subsequently applied to a bimorph beam which has electrostrictive wafer mounted on the top as well as bottom surfaces of the beam. The bimorph beam is chosen so that we can control the beam by applying different and independent electric potentials for the top and bottom surfaces of the beam. The non linearity in the constitutive model results in the generation of additional nonlinear waves. The formulated spectral element elegantly captures these nonlinear waves. The efficiency of electrostrictive actuator for both single and multi modal open loop control is studied in detail. This aspect is demonstrated through a number of numerical examples. The studies also show that the control efficiency in electrostrictive actuator is limited by the amplitude of the electric field. When the electric field is beyond a certain level, it can be seen that although the amplitude of the primary wave decreases, non-linearity in the model causes sign reversal and splitting of incident and reflected waves. It can also be seen that the amplitude of nonlinear wave tend to increase with increase in applied electric field. The paper initially investigates the use of electrostrictive material as an actuator and this idea is gradually expanded to the use of electrostrictive material as a sensor. Various numerical experiments are carried out to demonstrate this aspect and its applications in the detection of the crack. It can be seen that the spectral element derived elegantly captures the location and size of the crack.

A Multifunctional Control Strategy for Oscillatory Current Sharing in DC Microgrids

In this paper, a multifunctional control strategy is proposed for an accurate sharing of oscillatory and dc components of load currents among distributed generation (DG) units in a dc microgrid. The proposed controller is composed of a voltage control loop and a multiloop current control unit. The voltage control loop is used to regulate the microgrid voltage as well as to share the dc component of load current through the conventional droop scheme. The current controller includes two main units to simultaneously (1) manage the oscillatory current sharing, (2) improve the stability of control unit, (3) diminish the voltage ripples, and (4) protect the DG units against overloads. In this paper, the idea of virtual conductance is introduced to modify the output impedance of converters for oscillatory current sharing among DG units. To restrict the instantaneous current of DG units under overload conditions, a maximum current limiter unit is introduced to the control system. The compensation signals of these units are eventually applied to the inner current control loop. This scheme guarantees the microgrid voltage to be maintained within the tolerable range under a variety of oscillatory conditions. The stability of the system is studied using small signal analysis. The proposed control scheme is examined in two different microgrid configurations of radial and ring structures. The effectiveness of the proposed method is evaluated through simulation studies.

Fractional order IMC based PID controller design using Novel Bat optimization algorithm for TITO Process

An independent design of adaptive multiloop fractional order internal model control based PID (FOIMC-PID) controller for a two input two output process (TITO) is presented. The multivariable system is decomposed by ideal decoupler to develop effective open loop transfer function (EOTF) and then it is approximated into first order plus dead time (FOPDT) model. The multiloop FOIMC-PID controller is designed for TITO model and controller parameters are optimally tuned using Novel Bat Optimization Algorithm (NBOA). The simulation results are demonstrated to show the effectiveness of proposed controller and also some tuning procedure is suggested to get better FOIMC-PID controller performance.

MPC for Robot Manipulators With Integral Sliding Modes Generation

This paper deals with the design of a robust hierarchical multiloop control scheme to solve motion control problems for robot manipulators. The key elements of the proposed control approach are the inverse dynamics-based feedback linearized robotic multi-input-multi-output (MIMO) system and the combination of a model predictive control (MPC) module with an integral sliding mode (ISM) controller. The ISM internal control loop has the role to compensate the matched uncertainties due to unmodeled dynamics, which are not rejected by the inverse dynamics approach. The external loop is closed relying on the MPC, which guarantees an optimal evolution of the controlled system while fulfiling state and input constraints. The motivation for using ISM, apart from its property of providing robustness to the scheme with respect to a wide class of uncertainties, is also given by its capability of enforcing sliding modes of the controlled system since the initial time instant, allowing one to solve the MPC optimization problem relying on a set of linearized decoupled single-input-single-output (SISO) systems that are not affected by uncertain terms. The proposal has been verified and validated in simulation, relying on a model of a COMAU Smart3-S2 industrial robot manipulator, identified on the basis of real data.

Event-triggered sliding mode control of stochastic systems via output feedback

This paper is concerned with event-triggered sliding mode control (SMC) for uncertain stochastic systems subject to limited communication capacity. We consider the stochastic perturbation, exogenous disturbance and the network-induced communication constraints in a whole framework of SMC design. The measurable output is adequately sampled for periodic computation of a designed event trigger. Network-induced delays are characterized in the transmission over a shared network communication from sensor to controller. An event-triggered zero-order-hold (ZOH) is employed to keep receiving and sending the delayed measurement for control updating, and a state observer is designed to estimate the system state and to facilitate the design of sliding surface. Then, based on the measurement and observer’s outputs, a novel SMC law is presented. The stochastic stability for the overall closed-loop system is analyzed, and the exogenous disturbance is attenuated in an H∞ sense. Furthermore, the presented event-triggered SMC methods are successfully applied to multi-loop control case considering shared communication limitation. Simulation results are provided to verify the effectiveness of the proposed design scheme.

Error-dependent data scheduling in resource-aware multi-loop networked control systems

In this work, we address the problem of event-based data scheduling for multiple heterogeneous LTI control loops over a shared resource-constrained communication network. We introduce a novel bi-character scheduling scheme, which dynamically prioritizes the channel access at each time-step according to an error-dependent priority measure. Given local error thresholds for each control loop, the scheduling policy deterministically blocks the transmission from sub-systems with lower error values. The scheduler then allocates the limited communication resource probabilistically among the eligible sub-systems based on a prioritized measure. We prove stochastic stability of the networked control system under the proposed scheduler in terms of f-ergodicity of the overall network-induced error. Uniform analytical performance bounds are further derived for an average cost function comprised of a quadratic error term and transmission penalty. The simulation results show that our approach results in a significant reduction of the aggregate network-induced error variance compared to the conventional scheduling protocols.

Optimization and control of a stand-alone hybrid solid oxide fuel cells/gas turbine system coupled with dry reforming of methane

This paper presents the hybrid solid oxide fuel cells (SOFC)/gas turbine (GT) system coupled with dry reforming of methane (DRM). The DRM is a syngas producer by consuming greenhouse gas. The stand-alone (off-the-grid) power system is developed by using a combination of a post-burner, recuperators and pressurized recycles in place of external energy supplies. To address the stand-alone operation and meet the complete combustion condition for the burner, the optimal operating conditions are initially determined by solving a constrained optimization algorithm for maximizing the hybrid power efficiency, and the dynamic control loops are implemented in a plantwide environment. In the proposed plantwide control strategy, the inventory control framework is added to regulate the plant component inventory, an air/fuel cross-limiting combustion control is added to ensure complete combustion and reduce heat loss, and the power and CO2 emission control configuration is added to achieve the quality control performance. Finally, the simulation shows that the IMC-based multi-loop control scheme can efficiently regulate the total system power and control CO2 emissions per kWh of electricity as well.

A simplified time-domain design and implementation of cascaded PI-sliding mode controller for dc–dc converters used in off-grid photovoltaic applications with field test results

A time-domain design methodology for voltage regulation control of dc–dc boost and buck-boost converters based on a multi-loop controller with PI regulator for the outer loop and an inner loop with sliding mode current controller has been developed for renewable energy applications such as photovoltaic (PV)-fed dc–dc converters. This paper proposes a new method for the design of PI regulators in such multi-loop control scheme. The proposed design presents a simple analytical method for selecting controller gains and has been validated by simulation as well as hardware implementation. Also, this paper presents an illustrative example based on the proposed design for the voltage regulation control of PV-fed boost converters for off-grid applications. The simulation results for varying irradiation, temperature and load along with stability analysis have been presented in this paper. The proposed controller is implemented in hardware for a 1.1 kW PV-array-fed boost converter. Performance analysis based on field test results using real-time weather data validates the proposed design. Therefore the proposed controller could be considered as an attractive solution for off-grid renewable energy applications like PV- or fuel-cell-fed dc–dc converter, where the variations are stochastic in nature.

Multifunctional Control Strategy for Asymmetrical Cascaded H-Bridge Inverter in Microgrid Applications

A multifunctional control strategy for a single-phase asymmetrical cascaded H-bridge multilevel inverter (ACHMI), suitable for microgrid systems with nonlinear loads, is presented. The primary advantage of ACHMI is to produce a staircase output voltage with low harmonic content utilizing unequal dc voltages on the individual H-bridge cells. In a grid-connected mode of operation, the control strategy of the ACHMI is based on the conservative power theory, providing selective disturbing current compensation besides injecting its available energy. In autonomous mode of operation, two different control methods along with a damping resistor in the filter circuit are developed for regulation of the ACHMI instantaneous output voltage in a variety of load conditions. The first method is a single-loop voltage control scheme without the need of any current measurement. The second one is a multiloop voltage control scheme with a load current feedforward compensation strategy and preservation of the grid-connected current control scheme. The steady-state response and stability of both voltage control schemes are analyzed, and based on the application requirement, the control schemes are implemented individually. The effectiveness of each control strategy is experimentally verified using a hardware-in-the-loop setup with the control algorithm implemented in the TMSF28335 DSP microcontroller.

Design of optical emission spectroscopy based plasma parameter controller for real-time advanced equipment control

Abstract With the advent of more than Moore’s law era, control of plasma etch process is expected to become inevitable. Given that highly complex plasma is a medium of etch processes, plasma parameters are key factors to be controlled. In addition, highly interactive plasma characteristics require multivariate control schemes. In this paper, we design a multi-loop controller which controls effective plasma parameters in Ar plasma conditions. The effective plasma parameters obtained by optical emission spectroscopy are paired with plasma reactor instrumental variables through relative gain array and singular value decomposition. Each single input-single output (SISO) system based on the pairing result shows successful disturbance rejection performance but interactions between SISO controllers occur. In order to handle the interactions, 2 × 2 multiple input-multiple output (MIMO) controllers with and without decouplers are simulated to track set point change. Based on the simulations, a MIMO controller with decouplers is implemented in a capacitively coupled plasma reactor and show feasible control performance without interaction. Hopefully, the results introduced in this paper contribute to making progress in plasma parameter control.

Multi‐loop virtual synchronous generator control of inverter‐based DGs under microgrid dynamics

The increasing penetration of small-scale renewable power sources into the passive distribution networks brings direct deteriorated impacts on operation of microgrids due to the lack of inertia and damping property, e.g. unexpected voltage rise, increased fault level. To this end, this study presents a novel virtual synchronous generator (VSG) based multi-loop control solution aiming to efficiently manage the embedded inverter-based renewable distributed generators (DGs) in a plug-and-play fashion. With such multi-loop control structure, the virtual flux oriented control is adopted to avoid the harmonic interference and guarantee accurate control of vector orientation process in the U / I closed-loop feedback based vector control. The VSG based control is able to obtain timely current response and carry out static error free tracking control of output voltage. The pre-synchronising control aligned to the outer loop to ensure smooth transition from island to grid-connected mode is also studied. The proposed control solution is assessed through a set of simulation experiments in a microgrid for both island and grid-connected scenarios, and the numerical result confirms its technical benefits.