Simple Control Structure Research Articles

Model predictive flexible power control for grid‐connected inverter under unbalanced network

Model predictive power control (MPPC) is considered as a promising algorithm utilised in grid-connected inverter due to its fast dynamic response, simple control structure and multi-objective optimisation. However, under unbalanced network, the grid current with excessive distortion troubles the application of MPPC. Based on the conventional MPPC, this study proposes a model predictive flexible power control (MPFPC) to reduce the current total harmonic distortion and achieve three flexible targets, including the cancellation of the active- and reactive-power oscillation and the negative-sequence grid current. In this algorithm, the power references are divided into two parts, which are used as reference values for the instantaneous active power control (IAPC) and the instantaneous reactive power control (IRPC), respectively. The three flexible targets are achieved by regulating the ratio of the two parts of power reference. Meanwhile, the grid current of MPFPC is the sum of IAPC and IRPC; therefore, the grid current distortion can be suppressed effectively. Simulation and experimental platform constructed by a three-level inverter are established to validate the effectiveness of MPFPC, and the results exhibit that the MPFPC meets the above expectations.

Model predictive DC-component power control for grid-connected inverters under unbalanced network

Under an unbalanced network, model predictive control (MPPC) with a new definition of the instantaneous reactive power has attracted a great deal of attention due to its simple control structure and outstanding steady-state performance. However, the reactive power cannot precisely trace the nonzero reference. In this paper, a model predictive DC-component power control (MPDCPC) is proposed to tackle the above-mentioned problem. Additionally, the MPDCPC can eliminate oscillations on the reactive power and the negative-sequence current. Then, the corresponding mathematical formulas are derived to extract the DC-component powers, to calculate the DC-component power derivative and to modify the power reference. By regulating the DC-component power to trace the modified reference, the MPDCPC can reduce the current harmonic and remove the oscillations on the active power, reactive power or negative-sequence current. Simulation and experimental platforms are established to demonstrate the validity of MPDCPC. Results demonstrate that the MPDCPC can effectively suppress current distortion and remove oscillations on active power, reactive power or negative-sequence current. In addition, the MPDCPC can successfully break through the restrictions of the MPPC with a new definition of instantaneous reactive power.

PI Controller Tuning Based on Stochastic Optimization Technique for Performance Enhancement of DTC Induction Motor Drives

Direct torque-controlled (DTC) induction motor (IM) drives over the years have been the work force of industries. The popularity of this motor drive is due to the low cost and low maintenance of induction motor coupled with the fast dynamic response and simple control structure of direct torque control method. The robust performance of the DTC induction motor drive depends on the proper tuning of its speed controller. The proposed paper makes use of the stochastic optimization technique, namely the popular harmony search algorithm, and is compared with the parameter-free Jaya algorithm for tuning the gains of the speed proportional integral controller. Simulation studies in MATLAB/Simulink show the success of the Jaya optimization for improving the performance of DTC drive with respect to speed and torque peak overshoot and steady-state error under different drive operating conditions.

Fractional-Order Surge Control of Active Magnetic Bearings Suspended Compressor

H∞ surge control in centrifugal compressors by using active magnetic bearings (AMBs) has been successfully designed and implemented. However, the structure and design process of H∞ surge control are quite complex. This paper reports on the design and implementation of fractional-order proportional-derivative control (FOPD) that results in the required specifications of surge control with a simple controller structure. To validate its effectiveness, the proposed FOPD surge controller has been implemented on a centrifugal compressor test rig equipped with AMBs. Simulation and experimental results show that the FOPD surge controller outperforms integer-order PD (IOPD) control in extending the surge limit in terms of the mass flow and provides similar performance as the H∞ controller.

Speed Regulation Control for an Integrated Motor-Transmission System under External Disturbances

Precise motor speed regulation control is essential to achieve a good gear shifting quality of the integrated motor-transmission (IMT) system, in which the relative speed between outgoing shaft and the gearwheel to be engaged can be eliminated directly through regulation of the motor speed. The speed regulation control confronts the difficulty that there exist external disturbances on the motor shaft, like the unknown load torque arised from bearing friction, oil shearing and oil churning, etc. To deal with these disturbances, a robust speed regulation controller combined a nominal proportional control and integral sliding mode control is proposed. The former is designed to achieve a good speed tracking performance and the latter provides functionality of disturbances rejection. The effects of different controller parameters for the robust controller design are assessed via simulations. Moreover, to verify the effectiveness of the combined control scheme in practical engineering use, experiments are carried out on a test bench with a real IMT powertrain system. Results indicate that the proposed approach can attain a rapid and smooth speed regulation process with a simple controller structure and good robustness.

Adaptive Feedback Control for a Pasteurization Process

The milk pasteurization process is nonlinear in nature, and for this reason, the application of linear control algorithms does not guarantee the obtainment of the required performance in every condition. The problem is here addressed by proposing an adaptive algorithm, which was obtained by starting from an observer-based control approach. The main result is the obtainment of a simple PI-like controller structure, where the control parameters depend on the state of the system and are adapted online. The proposed algorithm was designed and applied on a simulated process, where the temperature dependence of the milk’s physical properties was considered. The control strategy was tested by simulating different situations, particularly when time-varying disturbances entered the system. The use of the adaptive rule reduces the variance generally introduced by the PI or PID controller.

Decentralised control method of battery energy storage systems for SoC balancing and reactive power sharing

Battery energy storage systems (BESSs) are important for the operation and optimisation of the islanded microgrid (MG).However, the BESSs will have different dynamics due to the differences in characteristics and operating conditions, leading to unbalanced state-of-charges (SoCs). On the other hand, because of the unequal line impedances, the conventional reactive power-voltage (Q–V) droop control results in disproportional reactivepower-sharing. To deal with suchissues, a power -frequency (P–f) droop-based decentralised SoC balancing strategy is developed first to regulate the active power outputs of BESSs in terms of SoCs, thus ensuring SoC balancing among BESSs. Furthermore, the authors develop a Q–V droop-based decentralised reactive power-sharing strategy to dispatch the reactive power among BESSs in terms of their respective reactive power ratings. In this way, compared with the existing control strategies, the proposed control method is fully decentralised, which removes the necessity for both the central controller and communication links. Moreover, the proposed control method has a simpler control structure with the plug and play capability. Finally, the simulation results are presented in an islanded MG to demonstrate the effectiveness, plug and play capability of the proposed control method.

Model Predictive Control-Based Virtual Inertia Emulator for an Islanded Alternating Current Microgrid

Conventional primary control employs outer-loop droop and inner-loop cascaded linear control to realize local voltage regulation and power-sharing of an islanded ac microgrid. However, it has a complex structure, limited dynamic response, and a rapid rate of change of frequency when disturbances occur. This article resolves these issues by proposing a model predictive control-based virtual synchronous generator (VSG-MPC). An improved finite-set MPC is first proposed for the inner loop, achieving simplified control structure, faster dynamic response, enhanced bandwidth and stability, as well as improved current limitation. In the outer control loop, a simplified VSG without a phase-locked loop is employed to realize active power-sharing and inertia emulation. The merits above are verified by a description function of MPC and the frequency-domain response of the overall VSG. Simulation and experimental results verify the feasibility of the proposed method.

A Switching Control Strategy Based on Switching System Model of Three-Phase VSR Under Unbalanced Grid Conditions

The power electronic converters are typical nonlinear systems. The conventional linearized modeling methods cannot accurately describe the working process of the converter. The control strategies based on the linearized model can only achieve good performance near the working point. By considering the switching nonlinear system nature of the converters, the main contributions of this article are as follows: a current error switching model is developed for the three-phase voltage source rectifiers (VSRs) under unbalanced grid for the first time; and an improved stability analysis theorem is proved for the switching control of the three-phase VSRs under unbalanced grid condition. These contributions are used to design a simple current switching controller for VSRs. The main advantages of the proposed modeling and control strategy are as follows. The switching model of the converter is built without any approximation. It accurately describes the physical working principle of the three-phase VSR under unbalanced grid condition. The proposed current switching controller has a very simple control structure. The switching states of the converter are directly produced according to the switching rule. There are no control parameters, no complicated rotation transformation, and no voltage vector modulation in the proposed switching controller. The proposed switching controller is not sensitive to the circuit parameters. The robustness of the control system is improved without any extra compensators or observers. Simulation and experiment verifies the correctness, the effectiveness, and the superiority of the proposed theory and methodology.

Rotor Field-Oriented V/ f Drive System Implementation With Oscillation Suppression Compensator in Induction Motors

Unlike a sensorless vector drive, because a volts per hertz ( V/f) drive has a simple controller structure, this drive is extensively used for low-cost drive markets that target applications, such as fan, pump, and blower drives, which do not require high performance. For an induction motor's V/f drive system, a voltage boost enables correct flux excitation. Moreover, to decrease the steady-state speed error, slip compensation is required. Furthermore, an oscillation suppression compensator (OSC) may be added to improve the system's stability and dynamic performance. In this article, a rotor field-oriented V/f drive implementation is presented. Both voltage boost and slip compensation strategies based on rotor field orientation are introduced, and a novel OSC design and stability analysis are presented with the numerical analysis of the root locus and stability surface. Finally, regarding the various practical situations of the induction motor, the proposed V/f drive system's effectiveness is confirmed by simulations and experiments.

Online Nonlinear Dynamic System Identification With Evolving Spatial–Temporal Filters: Case Study on Turbocharged Engine Modeling

Vehicle control problems have been historically using simple control structures with lookup tables that were designed to use vehicle steady-state characterization to calibrate and are suited better for older systems with lower complexity/dimension. However, the increasing complexity of the vehicle systems and stringent regulations requires optimal control of the dynamic vehicle systems. In this brief, we present an efficient online identification methodology with a composite local model structure that will be critical to the new control paradigm. We first introduce the concept of evolving spatial–temporal filters (STFs) that dynamically transform an incoming input–output data stream into a nonlinear combination of local models. The local models are weighed by an array of weights corresponding to the compatibility of the input–output data to a set of ellipsoidal clusters that partition the input–output space. The filters exploit ellipsoidal-shape evolving clusters as function bases and a distance metric defined as a combination of Mahalanobis distance and scaled local model prediction error. Parameters of the filters and local models can be updated simultaneously online, making adaptive optimization a possibility for vehicle systems. Evolving clustering and recursive least square techniques are exploited to simultaneously update the cluster parameters and local linear models. We apply the developed algorithm to the modeling of a turbocharged internal combustion engine that is a highly nonlinear and complex system. Promising performance is demonstrated both in a high-fidelity simulator and on an experimental vehicle.

Photovoltaic-Wind and Hybrid Energy Storage Integrated Multisource Converter Configuration-Based Grid-Interactive Microgrid

In this article, a new dc–dc multisource converter configuration-based grid-interactive microgrid consisting of photovoltaic (PV), wind, and hybrid energy storage (HES) is proposed. Control structure along with power sharing scheme to operate the system under various operating modes, such as: 1) grid-connected mode; 2) islanded mode; 3) state of charge of battery ( $SoC_{b}$ ) less than or greater than specified limits; and 4) operating renewable sources (PV and wind) at maximum power point, is presented. The detailed analysis, modeling, and design of the proposed configuration and control structure are presented. The key highlights of the proposed configuration are: 1) low component count; 2) voltage boosting, voltage regulation of supercapacitor and power-sharing among the battery and supercapacitor are inherent; and 3) simple control structure with a reduced number of sensors. Supercapacitor-battery-based HES is interfaced which effectively handle the power fluctuations due to the wind, PV, and sudden load disturbances. Integration of supercapacitor to respond to high-frequency fluctuations increases the lifetime of battery storage and reduces the sizing of the storage unit. The proposed system is verified through digital simulation and experimental results.

Photovoltaic-Wind and Hybrid Energy Storage Integrated Multi-Source Converter Configuration for DC Microgrid Applications

In this paper, a new multi-source and Hybrid Energy Storage (HES) integrated converter configuration for DC microgrid applications is proposed. Unlike most of the multi-input converter configurations, a supercapacitor-battery based HES is interfaced which effectively handle the power fluctuations due to the wind, photovoltaic and sudden load disturbances. Integration of supercapacitor to respond for high-frequency fluctuations increase the lifetime of battery storage and reduce the sizing of the storage unit. The control structure is framed to achieve power balance in the system along with basic functionality such as operating renewable sources (PV and wind) at maximum power point and charging and discharging of energy storage based on power availability. The key highlights of the proposed configuration are: (i) Less number of switches, (ii) Voltage boosting, voltage regulation of supercapacitor and power-sharing among battery and supercapacitor are inherent, (iii) Simple control structure with a reduced number of sensors. The detailed analysis, modeling, and design of the proposed configuration and control structure are presented along with MATLAB simulations and experimental validations.

A Bumpless Transfer Control Strategy for Switched Systems and Its Application to an Aero-Engine

This article investigates a bumpless transfer control problem for switched systems. Since the control input signal of a controller switching system jumps when a switching occurs, the transient performance of the practical system may suffer bad effects. To avoid the damage to the system caused by the jump of control input signal, we propose a simple bumpless transfer control structure containing a differentiator and an integrator to smooth the control input signal and a common compensator designed to compensate the deviation from the original switching control input signal for eliminating the steady-state error. Furthermore, we give the sufficient conditions to guarantee asymptotic stability for linear and nonlinear bumpless transfer control systems, respectively. Moreover, we verify the recoverability and effectiveness of the proposed control strategy. Finally, a hardware-in-the-loop simulation for a variable cycle engine shows the feasibility and superiority of the proposed method.

Positioning Control of an Antagonistic Pneumatic Muscle Actuated System using Feedforward Compensation with Cascaded Control Scheme

This paper presents a feedforwardcompensation with cascaded control scheme (FFC) for the positioning control of a vertical antagonistic based pneumatic muscle actuated (PMA) system. Owing to the inherent nonlinearities and time varying parameters exhibited by PMA, conventional fixed controllers unable to demonstrate high positioning performance. Hence, the feedforward compensation with cascaded control scheme is proposed whereby the scheme includes a PID controller coupled with nonlinear control elements. The proposed scheme has a simple control structure in addition to its straightforward design procedures. Though there are nonlinear control elements involved, these elements are derived from the open loop system responses that does not requires any accurate known parameters. Performance of the FFC scheme are then evaluated experimentally and compared to a PID controller with feedforward compensation (FF-PID) in point-to-point motion of different step heights. Overall, the experimental results show that the effectiveness of the proposed FFC scheme in reducing the steady state error to zero in comparison to FF-PID controller for all cases of step heights examined.

Analysis and design of single‐stage, two‐mode AC/DC converters for on‐board battery charging applications

On-board battery chargers (OBCs) offer the feasibility of charging electric vehicles or plug-in hybrid electric vehicles with a standard household power supply. In this study, an OBC using two-mode topology (step-up or step-down) is presented. One of the main concern with two-mode topologies is the hard transitions of the converter operating from boost mode to buck mode or vice versa, which can deteriorate the performance of converter. This study provides smooth transitions between the modes and can have an output that could be more or less than the peak of input voltage. The detailed analysis of operating modes involved in the converter are analysed in the study. Moreover, the stress analysis on the components is also detailed in this study and the results are compared with existing two-mode converters. A simple dual-loop control structure with the proposed modulation technique eliminates the sub-harmonic oscillations and stabilizes the output voltage is also presented in the study. Experimental validation is performed for a 1.0 kW laboratory prototype and the results obtained are detailed in the study. This proposed converter topology can be applied as a single-stage OBC or as a power factor correction converter with variable DC link voltages in isolated two-stage OBC.

Control Design of a Swarm of Intelligent Robots: A Closed-Form H2 Nonlinear Control Approach

A closed-form H2 approach of a nonlinear trajectory tracking design and practical implementation of a swarm of wheeled mobile robots (WMRs) is presented in this paper. For the nonlinear trajectory tracking problem of a swarm of WMRs, the design purpose is to point out a closed-form H2 nonlinear control method that analytically fulfills the H2 control performance index. The key and primary contribution of this research is a closed-form solution with a simple control structure for the trajectory tracking design of a swarm of WMRs is an absolute achievement and practical implementation. Generally, it is challenging to solve and find out the closed-form solution for this nonlinear trajectory tracking problem of a swarm of WMRs. Fortunately, through a sequence of mathematical operations for the trajectory tracking error dynamics between the control of a swarm of WMRs and desired trajectories, this H2 trajectory tracking problem is equal to solve the nonlinear time-varying Riccati-like equation. Additionally, the closed-form solution of this nonlinear time-varying Riccati-like equation will be acquired with a straightforward form. Finally, for simulation-controlled performance of this H2 proposed method, two testing scenarios, circular and S type reference trajectories, were applied to performance verification.

Simulation and experimental comparative analysis of the DC-DC converter topologies for wind driven SEIG fed DC nanogrid

Wind energy conversion system (WECS) based DC nanogrid topology is proposed in this paper. The WECS topology is based on self-excited induction generator (SEIG) which is cost effective, and has rugged construction and combination of diode bridge rectifier (DBR) with DC-DC converter to implement the maximum power point tracking technique. The combination of diode DBR with DC-DC converter allows simple control structure due to one controllable switch as compared to complex control in the conventional controlled/semi-controlled rectifier with six controllable switches. Further, proposed DC nanogrid topology enhances the SEIG efficiency due to the reduction of harmonic injection on machine side from DBR implementation. The detailed harmonic spectrum analysis is carried out by developing the laboratory scale prototype of DBR based SEIG topology for WECS to justify the selection of DBR instead of the controlled rectifier. Further, to analyze the effect of different types DC-DC converter to make combination with the DBR. Afterwards, the dynamic performance of the proposed DC nanogrid topology is analyzed to understand the selection of the DC-DC converter with DBR for different DC grid voltage conditions.

Traffic Control on Freeways Using Variable Speed Limits

A new practical oriented feedback control structure for traffic control on freeways using variable speed limits is presented. Therefore, a simple controller structure which satisfies legal and operator demands for a single road is derived from system analysis of a macroscopic traffic model. Furthermore, this road-controller is accompanied with a sequence control and a road-network-controller. In contrast to existing work, novel control variables are used within controller design and tuning guidelines for non-control-engineers are given. The proposed control structure is investigated in simulations, which have been validated using experimental data from the German autobahn.

Optimal PID and Antiwindup Control Design as a Reinforcement Learning Problem

Deep reinforcement learning (DRL) has seen several successful applications to process control. Common methods rely on a deep neural network structure to model the controller or process. With increasingly complicated control structures, the closed-loop stability of such methods becomes less clear. In this work, we focus on the interpretability of DRL control methods. In particular, we view linear fixed-structure controllers as shallow neural networks embedded in the actor-critic framework. PID controllers guide our development due to their simplicity and acceptance in industrial practice. We then consider input saturation, leading to a simple nonlinear control structure. In order to effectively operate within the actuator limits we then incorporate a tuning parameter for anti-windup compensation. Finally, the simplicity of the controller allows for straightforward initialization. This makes our method inherently stabilizing, both during and after training, and amenable to known operational PID gains.