Single Control Structure Research Articles

Flexible and Advanced DBS: A distributed control strategy for a DC[sbnd]Nanogrid

Nowadays, it is well known that the energy system is going towards a clear and necessary energy transition. New enabling technologies are needed to accelerate this transition, like the dc nanogrids. The dc nanogrids are designed for a bottom-up approach and play an important role due to their scalability and modularity features. To take advantage of these features, in this paper, the authors propose a flexible and advanced DBS distributed control strategy so to be independent from the dc nanogrid configuration and operating both in stand-alone and in the grid-connected mode. At this scope the dc nanogrid is modelled as the connection of multiple generic nodes and a hierarchical control architecture is applied as a single control structure. It facilitates the seamless transition between the operating modes, independently from the dc nanogrid configuration. The effectiveness of the proposed flexible and advanced DBS is validated by some experiments performed on a testbed integrating a dc nanogrid laboratory prototype.

A Unified Active Frequency Regulating and Maximum Power Point Tracking Strategy for Photovoltaic Sources

In order to optimize the extraction of solar energy, photovoltaic sources are commonly operated under the control of the so-called maximum power point (MPPT) strategy. However, as the rate of PV installations increases explosively, traditional MPPT algorithms may cause problems such as frequency deviation and power fluctuations, making system frequency stability a challenge due to the inherent intermittent and stochastic nature of PVs. Consequently, in order to reduce the investment and maintenance costs of storage systems, innovative control is expected for PV sources to provide ancillary services for the system, especially for weak systems such as microgrids. In this paper, a novel active power control (APC) strategy, based on characteristic curve fitting, is proposed to flexibly regulate the PV output power. The transient process performance and robustness of the system are improved with the proposed APC strategy. In conjunction, an f–P droop mechanism is designed to provide a frequency regulating (FR) service for the AC microgrid. The comprehensive control strategy unifies the FR function with the traditional MPPT function in a single control structure, allowing the PV source to operate either in the MPPT mode when the system frequency is nominal or in FR mode when the frequency exceeds it. The transition between MPPT and FR is autonomous and fully decentralized, which improves the PV generation efficiency as well as ensuring generation fairness among different parallel PV sources. Importantly, the proposed control strategy does not require any internal bundled energy within the PV generation system to achieve FR capability, but it effectively collaborates with the system-level energy storage system, thus reducing the necessary battery capacity. A detailed dynamic model of a PV generation system is constructed to validate the feasibility and effectiveness of the proposed control strategy.

Modeling and Centralized-ZVS Control for Wireless Charging Electric Vehicles Supplied by Parallel Modular Multi-Inverters

In this paper, a parallel modular multi-inverter (PMMI) topology is proposed to supply high power for wireless charging electric vehicles (EVs). A major challenge in the implementation of PMMI topology is zero-voltage switching (ZVS) for all inverters to avoid high-frequency switching losses. Therefore, a centralized-ZVS control and master–slave frequency following (MSFF) strategy are presented to realize ZVS for all PMMIs by a single controller structure without extra controllers needed on the slave inverters. Meanwhile, a modeling method directly related to the ZVS angle and operating frequency for an arbitrary number of PMMIs is proposed and linearized to analyze the system dynamic characteristics at the operating point. Additionally, to obtain the desired dynamic performance, an optimal controller coefficient (OCC) configuration method is proposed for the design of controller parameters. Finally, a laboratory wireless power transfer (WPT) prototype supplied by three PMMIs is designed, built, and tested to verify the correctness of the theoretical analysis. Experimental results show that the ZVS angle of all PMMIs can maintain at the preset value with the desired settling time under designed the PI controller parameters. The measured whole system power transmission efficiency is 94.1% at a 10 Ω load.

Aggressive maneuvering of a quadcopter via differential flatness-based fuzzy controllers: From tuning to experiments

In recent years, the rapid development in contemporary technology has brought nano quadcopters with high agility. This paper presents a new differential flatness-based Single Input Fuzzy Logic Controller (SFLC) structure for aggressive maneuvering control alongside its real-world application on Crazyflie 2.1 nano quadcopter. We propose both Type-1 and Interval Type-2 SFLCs as the primary controllers in the flight control system, which are built on the concept of differential flatness. We investigate how the design parameters of SFLCs shape the characteristics of the fuzzy mapping through a geometric approach by analyzing the region and level of aggressiveness/smoothness. Based on the analysis, we present simple tuning guidelines and then design fuzzy logic-based flight control systems, which were implemented as onboard real-time controllers. Finally, we evaluate the performance of SFLCs in comparison with their crisp differential flatness-based nonlinear counterparts for four trajectories with distinct dynamics and shapes in the real world. The presented comparative experimental results clearly show the performance improvements when the proposed T1 and IT2 SFLCs are deployed for real-time aggressive maneuvering.

Positioning control of liquid crystal elastomer actuator based on double closed-loop system structure

This paper proposes a control strategy with a double closed-loop system structure to achieve the positioning control of the photo-responsive liquid crystal elastomer (LCE) actuator. This work helps developing photo-responsive soft robot applications that are totally different from conventional electro-driven ones. In the proposed strategy, the outer loop is a combination of a PID controller in parallel with a feed-forward controller, the PID controller ensures that the positioning control target can be reached, and the feed-forward controller is an inverse model of the temperature-deformation relationship of the LCE actuator developed recently, to calculate the target temperature of the LCE actuator. The inner loop is a PID controller, which ensures that the temperature of the LCE actuator converges to the target value calculated by the feed-forward controller. Parameters of the inner PID controller are tuned using a simulation-experiment combined method based on the Hammerstein–Wiener model, and parameters of the outer PID controller are tuned based on experiments. Experiments are performed to show the effectiveness of the proposed method, and comparisons are made with conventional single closed-loop control structure. Such a double closed-loop structure has faster response and higher accuracy, and it lays a basis for possible photo-responsive soft robot applications.

Analysis of Non-Minimum Phase System for AC/DC Battery Charger Power Factor Correction Converter

Electric mobility is nowadays one of the more important trends regarding pollution reduction and global warming due to fuel consumption. Big efforts are done in order to develop efficient and reliable power electronic systems for electric vehicles. In two stage on board-battery chargers, one way of improving efficiency is by means of ensuring the DC-DC isolated converter always operates in the nominal input/output voltage ratio, that could be achieved with a variable DC-link operation. In this paper, a four-switch buck-boost based AC/DC converter is deeply analyzed in order to improve its dynamic performance, the power factor and the total harmonic distortion. The converter suffers from a non-minimum phase characteristic in different input–output transfer functions, which reduces the closed-loop bandwidth of the system. Therefore, after a deep converter analysis has been done, different solutions have been evaluated and tested. Finally, a control to different output transfer functions of the converter become minimum phase, which allows us to increase the system bandwidth and, consequently, high power factor, low harmonics distortion, single control structure and fast dynamics for wide output voltage range are achieved.

Modulation of sagittal-plane center of pressure and force vector direction in human standing on sloped surfaces

The multi-joint coordination responsible for maintaining upright posture in the standing human manifests in the pattern of variation of the support-surface force (F). Assessment of both the translational and rotational kinematics in the sagittal-plane requires understanding the critical relationship between the direction and location of F. Prior work demonstrated that band-pass filtered F direction and center-of-pressure (CoP) covary in time such that the F vector lines-of-action pass near a fixed point called an intersection point (IP). The height of that IP (IPz) varies systematically with the frequency of the pass band. From F measurements in able-bodied humans (n = 17) standing on various pitched surfaces, the present study also found the emergent property of an IP, with IPz located above the center of mass (CoM) at frequencies <1.75 Hz and below the CoM for higher frequencies. This property aids in maintaining upright posture for various perturbation modes within a single control structure. From purely mechanical effects, standing on a pitched surface should not change IPz, however these measurements of F show that IPz is generally closer to CoM height. This characterization of quiet standing provides simple means of assessing the complex multi-joint coordination of standing and relates directly to the physical demands of controlling the translational and rotational aspects of body posture.

An Autonomous Control Scheme of Global Smooth Transitions for Bidirectional DC-DC Converter in DC Microgrid

In dc microgrid, energy storage system (ESS) plays a crucial role to provide short-or-long term and high-quality electric energy. The different control strategies for bidirectional dc-dc converter (BDC) of ESS in grid-tied and islanded modes pose challenges to the coordination control of the dc microgrid. This paper proposes an autonomous control scheme for the BDC in dc microgrid. The proposed control scheme is based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V^2-P$</tex-math></inline-formula> droop control and unifies bus voltage regulation and power regulation in a single control structure. Thus, global smooth transition between various operation modes can be achieved without any control strategy changes, which avoids various mode switch detection mechanisms and improves system stability. Furthermore, the proposed control scheme is fully decentralized which reduces the reliance on communication, and enhances the reliability of the microgrid. On the other hand, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V^2-P$</tex-math></inline-formula> droop approach eliminates the negative effect of widespread constant power loads (CPLs) in dc microgrid. The stability of the proposed control method is illustrated, and the design guideline of some critical control gains is addressed. Finally, the effectiveness of the proposed control scheme is validated by the real-time hardware-in-loop (HIL) platform.

Autonomous Control Strategy for Microgrid Operating Modes Smooth Transition

Microgrid transition between standalone and grid-connected modes is a promising alternative to provide the grid with increasing flexibility and availability. However, transition smoothness relies heavily on control topologies and corresponding parameters, which thus remains challengeable. Existing microgrid transition strategies have two major deficiencies: 1) Inverter control mode alters subjected to microgrid operating mode, for instance, the inverter in current control will switch to voltage control when microgrid disconnects to the utility grid; 2) Controller parameters are selected based on practice and experience, where a systematic and efficient approach does not exist. Motivated by these limitations, in this paper, an autonomous control strategy is proposed for microgrid smooth state transitions. It is highlighted in the following aspects: 1) The cascaded control strategy enables smooth state transition within a single control structure, which permits controller independent of mode switching; 2) Nonlinear-Simplex based algorithm is interfaced with electromagnetic transient simulations, searching for optimal controller parameters in order to minimize voltage deviation in a chain of microgrid events. The effectiveness of the control framework is validated with simulations in PSCAD/EMTDC and RTDS.

From standing balance to walking: A single control structure for a continuum of gaits

This article presents a control algorithm framework with which a bipedal robot can perform a variety of gaits by only modifying a small set of control parameters. The controller drives a number of variables, called non-emergent variables, to their desired trajectories resulting in a desired emergent walking gait. While the non-emergent variables remain the same independent of the gait, their desired trajectories are functions of a small set of control parameters that change as a function of the desired gait. This control algorithm has been tested on the humanoid robot COMAN, where different gaits including standing balance, stepping in place, periodic walking gaits with different velocities, as well as gait switching are demonstrated in experiments.

Low voltage ride-through capability improvement of microgrid using a hybrid coordination control strategy

This paper proposes a hybrid coordination control strategy to improve the low voltage ride-through (LVRT) capability of microgrids. During microgrid external failure, the overcurrent and the voltage sag of the microgrid can be effectively suppressed. Compared with traditional control strategies, the microgrid transient performance can also be enhanced by adopting a generalized control algorithm, performing the active switching of the energy storage (ES) operation mode and triggering the inductance-type flux coupling fault current limiters (FCLs). Among them, according to the hierarchical control structure of the grid-connected inverter, the generalized control algorithm is applied to ES. Through the two degrees of freedom's control principle and inverse plant modeling techniques, the generalized control algorithm can make the control layer of the ES inverter operate in all control targets (i.e., PQ control, droop control and Vf control), with a single control structure. Also, it eliminates the effects of microgrid distortion voltage and ES surge current on the control system of the ES inverter. For the inverter application layer, by introducing microgrid voltage and angular frequency feedforward compensation, the voltage and frequency fluctuation of the microgrid can be suppressed efficiently under the fault conditions. Therefore, with the help of the generalized control algorithm, the active switching function of ES can be carried out successfully, which is the key technology for microgrids to realize their operational mode smooth switching. Meanwhile, the voltage sag and overcurrent of microgrids can be significantly mitigated through generating reactive power from microgrids and injecting it into inductance-type FCLs with lower cost. The related theoretical derivation and simulation analysis under various scenarios (including asymmetric faults) confirm that the proposed control strategy can not only enhance the LVRT capability, but also strengthen the microgrid transient performance.

Design of a Novel Hybrid Control Strategy for ES Grid-Connected Inverter for Smooth Microgrid Transition Operation

A novel hybrid control strategy for energy storage (ES) is applied in accordance to the recommendations by the IEEE Std. 1676. The strategy mainly consists of the model predictive controller (MPC), two-degree of freedom algorithm and modified droop control. In inverter control layer, MPC and two-degree of freedom algorithm are adopted to construct a generalized controller to enable ES operate in microgrid grid-connected and islanded operation modes with a single control structure, motivating the smooth transition between microgrid operating modes. And MPC and two-degree of freedom algorithm are used in the control layer's current inner loop and voltage outer loop, respectively. Comparing with the traditional double loop control from the perspective of structure, with the novel strategy the control layer's current inner loop is without pulse width modulation (PWM) and only a single proportional-integral (PI) controller is applied in its voltage outer loop, so that ES's robust performance can be enhanced. And because of the high frequency inverter's equivalent transfer function with the smaller time constant, MPC's transfer function approximated as being unitary. Therefore, when the two-degree of freedom algorithm construct the inverse model for the controlled object in control layer's voltage outer loop, from the application layer perspective, the control layer can be recognized as unity gain. Thus, the control layer's harmful disturbances associated with the operation mode transitions can be removed. In the application layer, through adding decoupling inductance and introducing microgrid's voltage and angular frequency feedforward compensations to carry out PQ and modified droop controls, the grid-connected and island control targets of ES application layer can be realized. In particular, when microgrid operation mode is switched to island, with the help of modified droop control, microgrid's voltage and frequency can be quickly restored to standard values. Through the theoretical derivation and simulation results, it can be proved that under microgrid's plan or non-plan operation mode switching, the proposed method can not only can not only implement the microgrid smooth switching, but also strengthen its transient performance.

A New Fast Adaptive On-Time Control for Transient Response Improvement in Constant On-Time Control

These days, constant on-time current mode (COTCM) control scheme is widely used in the voltage regulator (VR) controllers because it has a higher light-load efficiency and a higher BW design capability while maintaining a simpler compensation requirement. One issue plaguing the COTCM control is its slow transient response which is caused by its fixed T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> operation. During the heavy-load step-up transient, the duty cycle becomes saturated and the inductor current increment becomes limited by T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> and the minimum off time (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF_MIN</sub> ) ratio, which can create a large undershoot at load step up. On the other hand, in the load step-down case, if the load step down occurs at the beginning of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> , a large overshoot can be created at the output. To solve this issue, this paper presents a method designed to increase the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> at load step up and then very quickly, decrease at load step down in order to reduce the undershoot and overshoot at output or otherwise save the output capacitor. In this proposed method, the increase or decrease of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> is proportional to the output change which eliminates the chance of any overcorrection or ring-back problem unlike the methods presented in prior forums. This feature enables the control to work seamlessly in a high-frequency load repetitive case in VR applications. Since this T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> change occurs only in the transient period when duty cycle is saturated, it does not affect the small-signal property of the COT control. Moreover, the proposed methods are very much compatible with the state-of-the-art single and multiphase COT control structures. Simulation and test results in both single and multiphase operations are also presented in the paper to verify the proposed concept.

Variable Admittance for pHRI: From Intuitive Unilateral Interaction to Optimal Bilateral Force Amplification

Abstract In this paper, a novel control architecture for physical human-robot interaction with a multi-degree-of-freedom (multi-dof) robotic manipulator is presented. An admittance regulation is used as a single control structure to effectively manage unilateral and bilateral interaction modes. This control approach is known to lead to intuitive manipulation for the operator during unilateral interactions. However, the effectiveness of this type of control for bilateral amplification on a rigid environment is often questioned. Here, variable admittance parameters are used in order to accommodate and optimize the system response to any potential dynamics. Three interdependent varying control laws are therefore presented, namely, a common unilateral variable admittance control, a bilateral gain scheduling control, and a continuous transition control. A 7-dof Kuka LWR 4 is used to conduct the experimentation and demonstrate the effectiveness of the control algorithms. A video showing different pHRI tasks with the novel control architecture is also provided.

Active Trailer Steering Control for High-Capacity Vehicle Combinations

In this paper, a new control strategy for the active steering of a trailers of longer and heavier vehicle combinations is proposed to improve both low speed maneuverability and high speed stability. A novelty of the approach is in the use of a single controller structure for all velocities using a gain scheduling method for optimal performance at any velocity. To achieve such a control objective, the problem is initially formulated as a path following problem and subsequently transformed into a tracking problem using a reference model. To support controller design, a generic nonlinear model of a double articulated vehicle, based on a single track model, is employed. The proposed systematic design approach allows to easily adjust the controller for additional trailers or different dimensions, in which only some of the towed vehicles are allowed to steer. The performance of the controller is verified on a high-fidelity multi-body model for evidencing the practical applicability of the approach. Simulation results show substantial reduction of both, the swept path width and tail swing for low speed, and the rearward amplification for high speed.

Single Neuron Stochastic Predictive PID Control Algorithm for Nonlinear and Non-Gaussian Systems Using the Survival Information Potential Criterion

This paper presents a novel stochastic predictive tracking control strategy for nonlinear and non-Gaussian stochastic systems based on the single neuron controller structure in the framework of information theory. Firstly, in order to characterize the randomness of the control system, survival information potential (SIP), instead of entropy, is adopted to formulate the performance index, which is not shift-invariant, i.e., its value varies with the change of the distribution location. Then, the optimal weights of the single neuron controller can be obtained by minimizing the presented SIP based predictive control criterion. Furthermore, mean-square convergence of the proposed control algorithm is also analyzed from the energy conservation perspective. Finally, a numerical example is given to show the effectiveness of the proposed method.

Vision-based navigation and guidance of a sensorless missile

The objective of this paper is to develop a vision-based terminal guidance system for sensorless missiles. Specifically, monocular vision-based relative navigation and robust control methods are developed for a sensorless missile to intercept a ground target maneuvering with unknown time-varying velocity. A mobile wireless sensor and actor network is considered wherein a moving airborne monocular camera (e.g., attached to an aircraft) provides image measurements of the missile (actor) while another moving monocular camera (e.g., attached to a small UAV) tracks a ground target. The challenge is to express the unknown time-varying target position in the time-varying missile frame using image feedback from cameras moving with unknown trajectories. In a novel relative navigation approach, assuming the knowledge of a single geometric length on the missile, the time-varying target position is obtained by fusing the daisy-chained image measurements of the missile and the target into a homography-based Euclidean reconstruction method. The three-dimensional interception problem is posed in pursuit guidance, proportional navigation, and the proposed hybrid guidance framework. Interestingly, it will be shown that by appropriately defining the error system a single control structure can be maintained across all the above guidance methods. The control problem is formulated in terms of target dynamics in a ‘virtual’ camera mounted on the missile, which enables design of an adaptive nonlinear visual servo controller that compensates for the unknown time-varying missile–target relative velocity. Stability and zero-miss distance analysis of the proposed controller is presented, and a high-fidelity numerical simulation verifies the performance of the guidance laws.

Design of a Generalized Control Algorithm for Parallel Inverters for Smooth Microgrid Transition Operation

A modified hierarchical control structure for distributed generation (DG) in microgrid is applied in accordance to the recommendations by the IEEE Std. 1676. In the inverter control layer, a generalized control algorithm is developed to enable the DGs to operate in all three modes (i.e., grid-forming, grid-feeding, and grid-supporting modes) with a single control structure, facilitating the seamless transition between the operating modes. This feature is obtained by designing the multiloop controller in the inverter control layer using inverse plant modeling techniques so that the dynamics of the inverter and the $LC$ filter are fully compensated, virtually transforming the controlled DG in the unity gain from the application layer perspective. Thus, effects of disturbances associated with the mode transitions are thus fully eliminated. The effectiveness of the proposed control algorithm is validated by numerical simulations and hardware-in-the-loop experiments. The results show that the DGs can fulfill the tasks defined in the system control layer with fast dynamics, desired accuracy, and good transient behavior.

Optimization and control for CO2 compression and purification unit in oxy-combustion power plants

High CO2 purity products can be obtained from oxy-combustion power plants through a CO2 CPU (compression and purification unit) based on phase separation method. To ensure that CPU (with double flash separators) can be operated under optimal conditions, this paper focuses on single variable analysis, multi-variable optimization, dynamic simulation and control system design for CPU in oxy-combustion power plants. It is found that optimal operating conditions are 30 bar, 30.42 °C, −24.64 °C, and −55 °C for multi-stage CO2 compressor discharge pressure, flue gas temperature after compression, first flash separator temperature, and second flash separator temperature, respectively. The designed double temperature control structure based on a systematic top-down analysis and bottom-up design method is more suitable than the single temperature control structure under different operating scenarios (load change and flue gas composition ramp change). To bear operating disturbances, operating strategies like elevating temperature, manipulating valves and adjusting setpoints are proposed. Influence of SOx would be more obvious than that of NOx, whilst the kij mixing parameters in Peng–Robinson property method affects little on process optimization and control system design. Comprehensive dynamic model with specified control system provides possibility to integrate CPU with full-train oxy-combustion power plants.

Design and Control of an Extractive Distillation System for Benzene/Acetonitrile Separation Using Dimethyl Sulfoxide as an Entrainer

A method for benzene/acetonitrile separation using extractive distillation is presented, and dimethyl sulfoxide (DMSO) is selected as the entrainer. Steady state and dynamic simulations for this process are implemented by commercial simulators (Aspen Plus and Aspen Dynamics). On the basis of global economic optimization, a design with optimized operation conditions for this process is developed. Two single temperature control structures with fixed reflux ratio or fixed reflux-to-feed ratio and a dual-end temperature control structure for entrainer recovery column are presented to evaluate the dynamic performance with feed flow rate and feed composition disturbances, and the last structure is quite effective.