Control Design Specifications Research Articles

From Nonlinear Dominant System to Linear Dominant System: Virtual Equivalent System Approach for Multiple Variable Self-Tuning Control System Analysis

The stability and convergence analysis of a multivariable stochastic self-tuning system (STC) is very difficult because of its highly nonlinear structure. In this paper, based on the virtual equivalent system method, the structural nonlinear or nonlinear dominated multivariable self-tuning system is transformed into a structural linear or linear dominated system, thus simplifying the stability and convergence analysis of multivariable STC systems. For the control process of a multivariable stochastic STC system, parameter estimation is required, and there may be three cases of parameter estimation convergence, convergence to the actual value and divergence. For these three cases, this paper provides four theorems and two corollaries. Given the theorems and corollaries, it can be directly concluded that the convergence of parameter estimation is a sufficient condition for the stability and convergence of stochastic STC systems but not a necessary condition, and the four theorems and two corollaries proposed in this paper are independent of specific controller design strategies and specific parameter estimation algorithms. The virtual equivalent system theory proposed in this paper does not need specific control strategies, parameters and estimation algorithms but only needs the nature of the system itself, which can judge the stability and convergence of the self-tuning system and relax the dependence of the system stability convergence criterion on the system structure information. The virtual equivalent system method proposed in this paper is proved to be effective when the parameter estimation may have convergence, convergence to the actual value and divergence.

Robust feedback compensation and PID Tuning under actuator rate limit effect based on Bode’s integrals

The actuator rate limit is a common phenomenon in control systems while its underlying mechanism remains elusive. Magnitude reduction and phase delay generated by the rate limiter degrade the control performance, but this adverse effect has always been omitted in practical engineering. This study proposes a robust controller design method and controller update rules for general minimum-phase systems to compensate for the rate limit effect and improve control performance. Firstly, a simplified implementation method of “flat phase” constraint is carried out based on the Bode’s integrals for proportional–integral–derivative (PID) and fractional-order proportional–integral (FOPI) controllers. Secondly, the update rules of the controller design specifications are proposed based on Bode’s integrals to mitigate the actuator rate limit effect. The proposed method maintains the original controller structure without extra compensation filters, thereby can be easily applied to engineering practice. The outperformance of FOPI controllers compared to PID controllers is qualitatively analyzed and has been indicated to have lower probability of infinity loops under actuator rate limit. Illustrative examples and experimental results have demonstrated the effectiveness, robustness under gain variation and disturbance rejection capability of the proposed rate limit compensation strategies.

Design and Analysis of Nonlinear Controller for a Standalone Photovoltaic System Using Lyapunov Stability Theory

Abstract This paper presents the Lyapunov stability theory-based nonlinear controller design for a standalone photovoltaic (PV) system. The comparative analysis of different nonlinear controllers is also carried out. Due to the nonlinear I-V characteristics of photovoltaic systems, conventional hill-climbing methods like Perturbate and Observe and Incremental Conductance do not show reliable tracking of maximum power under various uncertainties. Therefore, these methods require nonlinear tools to meet control objectives and design specifications. Out of various nonlinear controlling techniques, the one presented in this paper is the sliding mode approach for maximum power point tracking (MPPT). In the context of Lyapunov stability theory, the sliding mode approach uses a switching manifold. In this approach, the system trajectories are made to follow the sliding surface and remain there forever to ensure the stability of equilibrium points. Two types of sliding mode controllers have been simulated, namely, conventional—sliding mode controller (CSMC) and terminal—sliding mode controller (TSMC). The results are analyzed and compared scientifically on various performance parameters including duty cycle ratio, ideal and PV output power, and time taken for error convergence, under varying dynamic conditions. All the control algorithms are developed in matlab/Simulink.

Sampled-Data Resilient Control for Hybrid Nonlinear Stochastic Systems Under Periodic DoS Attacks

This paper concerns the sampled-data resilient control for hybrid nonlinear stochastic systems under periodic denial of service (P-DoS) attacks. First of all, considering that the system is attacked by P-DoS and using the zero-order holder (ZOH) technique to further analyze the impact of such attacks on the control input. Secondly, via using the time-delay transform method, the attacked sampled-data resilient control system is modeled as a switched stochastic nonlinear time-delay closed-loop system. Thirdly, by using the piecewise Lyapunov functional method, the sufficient conditions to guarantee the mean square exponential stability of the attacked system are given. Then, according to these sufficient conditions, the controller parameters are solved when specific Lyapunov functional and controller design are given. Finally, an example is given to illustrate the efficiency of our results.

Applicability of an Active Back-Support Exoskeleton to Carrying Activities.

Occupational back-support exoskeletons are becoming a more and more common solution to mitigate work-related lower-back pain associated with lifting activities. In addition to lifting, there are many other tasks performed by workers, such as carrying, pushing, and pulling, that might benefit from the use of an exoskeleton. In this work, the impact that carrying has on lower-back loading compared to lifting and the need to select different assistive strategies based on the performed task are presented. This latter need is studied by using a control strategy that commands for constant torques. The results of the experimental campaign conducted on 9 subjects suggest that such a control strategy is beneficial for the back muscles (up to 12% reduction in overall lumbar activity), but constrains the legs (around 10% reduction in hip and knee ranges of motion). Task recognition and the design of specific controllers can be exploited by active and, partially, passive exoskeletons to enhance their versatility, i.e., the ability to adapt to different requirements.

Design of Fractional Order Controllers Using the PM Diagram

This work presents a modeling and controller tuning method for non-rational systems. First, a graphical tool is proposed where transfer functions are represented in a four-dimensional space. The magnitude is represented in decibels as the third dimension and a color code is applied to represent the phase in a fourth dimension. This tool, which is called Phase Magnitude (PM) diagram, allows the user to visually obtain the phase and the magnitude that have to be added to a system to meet some control design specifications. The application of the PM diagram to systems with non-rational transfer functions is discussed in this paper. A fractional order Proportional Integral Derivative (PID) controller is computed to control different non-rational systems. The tuning method, based on evolutionary computation concepts, relies on a cost function that defines the behavior in the frequency domain. The cost value is read in the PM diagram to estimate the optimum controller. To validate the contribution of this research, four different non-rational reference systems have been considered. The method proposed here contributes first to a simpler and graphical modeling of these complex systems, and second to provide an effective tool to face the unsolved control problem of these systems.

Control induced instabilities in fluidized bed spray granulation

This contribution is concerned with the stability problems occurring during the operation of continuous fluidized bed spray granulation processes with external sieve mill cycle. These processes are in general operated by a mass controller, which guarantees that the overall mass of particles in the granulation chamber stays in well-defined bounds. It is well-known that, depending on the milling diameter, instabilities may occur, which result in a nonlinear limit cycle of the particle size distribution. To overcome this problem two approaches have been proposed in the literature, constrain the admissible parameter space to exclude regions of instability and design additional stabilizing control loops. In the present contribution, the cause of this instability phenomenon will be studied. It will be shown that the instability is not inherent to the process, which turns out to be open-loop stable over the studied parameter range, but due to the mass controller. More specifically, it will be demonstrated that the zero dynamics of the granulation process become unstable for certain parameter ranges resulting in closed-loop unstable process behavior. To point out, that this behavior does not depend on the specific mass controller design procedure, three prototypical mass controllers of practical relevance are designed and analyzed.

Attitude trajectory planning and attitude control for quad-rotor aircraft based on finite-time control technique

Abstract The attitude control problem for quad-rotor spacecraft is investigated in this paper. Considering the yaw dynamical rate is slower than that of roll and pitch channel, firstly, we establish the attitude trajectory and then propose an attitude decoupled control strategy. For the specific attitude controller design, we design a finite-time attitude tracking controller based on the finite-time control technology. The rigorous stability analysis method based on homogeneous system theory is given, which proves that spacecraft attitude can reach the expected attitude in a finite time. Experimental results on a real quad-rotor platform show that the proposed control strategy has the advantages of convergence and strong disturbance rejection ability.

Robust output-feedback cascaded tracking controller for spatial motion of anisotropically-actuated vehicles

The paper presents the description, analysis, and validation of a cascaded trajectory-tracking control system designed for rigid-body vehicles moving in a 3-dimensional task space with non-banked maneuvering. In particular, we consider the anisotropically-actuated vehicles characterized by the presence of some privileged actuation directions and, simultaneously, actuation authority limitations along other directions. By combining the two specific control design methodologies, that is, the Vector-Field-Orientation (VFO) approach in the outer (kinematic-level) loop and the Active-Disturbance-Rejection (ADR) approach in the inner (dynamic-level) loop, we provide the resultant VFO-ADR control system characterized by an intuitive geometrical control strategy and robustness to a wide range of matched disturbances and substantial uncertainties of a vehicle model. To facilitate its practical applicability, the VFO-ADR control law has been designed in a way which requires feedback only from the positional configuration variables of a vehicle body. The results of experimental trials, obtained with the VFO-ADR control law applied to a laboratory-scale in-door airship, reveal its practical usefulness also in the case of underactuated vehicles under some specific motion conditions.

Finite-time stability and stabilization of nonlinear singular time-delay systems via Hamiltonian method

This paper investigates the finite-time stability (FTS) and finite-time stabilization for a class of nonlinear singular time-delay Hamiltonian systems, and proposes a number of new results on these issues. Firstly, an equivalent form is obtained for the nonlinear singular time-delay Hamiltonian systems by the singular matrix decomposition method, based on which some delay-independent and delay-dependent conditions on the FTS are derived for the systems by constructing a kind of novel Lyapunov function. Secondly, we use the equivalent form as well as the energy shaping plus damping injection technique to investigate the finite-time stabilization problem for a class of nonlinear singular port-controlled Hamiltonian (PCH) systems with time delay, and present a specific control design procedure for the systems. Finally, we give several illustrative examples to show the effectiveness of the results obtained in this paper.

Insights into the Oroville Dam 2017 Spillway Incident

In February 2017, a failure occurring in Oroville Dam’s main spillway risked causing severe damages downstream. A unique aspect of this incident was the fact that it happened during a flood scenario well within its design and operational procedures, prompting research into its causes and determining methods to prevent similar events from reoccurring. In this study, a hydroclimatic analysis of Oroville Dam’s catchment is conducted, along with a review of related design and operational manuals. The data available allows for the comparison of older flood-frequency analyses to new alternative methods proposed in this paper and relevant literature. Based on summary characteristics of the 2017 floods, possible causes of the incident are outlined, in order to understand which factors contributed more significantly. It turns out that the event was most likely the result of a structural problem in the dam’s main spillway and detrimental geological conditions, but analysis of surface level data also reveals operational issues that were not present during previous larger floods, promoting a discussion about flood control design methods, specifications, and dam inspection procedures, and how these can be improved to prevent a similar event from occurring in the future.

Robust Output Regulation of Counter-Flow Heat Exchangers

We consider a partial differential equation model widely used for counter-flow heat exchangers and the related robust output regulation problem with boundary control and boundary observation. We show that the control system is an exponentially stable regular linear system, which enables us to use a specific known controller design to robustly regulate the system. The results are illustrated with numerical simulations.

Tensor Product Model-based Robust Flutter Control Design for the FLEXOP Aircraft

Abstract This paper presents a flutter suppression control design methodology for the aircraft designed within the European research project, FLEXOP. The aim of the flutter suppression controller is to stabilize the aeroelastic modes and to extend the aircraft’s flutter free envelope. The first step is to develop a low order control oriented linear parameter varying (LPV) model of the aircraft. This is based on the bottom-up modeling approach. The key idea is to reduce the order of the subsystems that form the nonlinear aeroservoelastic model. A critical requirement of the low order model is to capture the flutter modes accurately. The second step is the control design, which is based on polytopic LPV representation. The Tensor Product (TP) type polytopic model of the aircraft is obtained with TP model transformation. TP model transformation in a numerical method based on the higher order singular value decomposition (HOSVD). It can generate various types of convex representations for LPV systems and offers a trade-off between the accuracy and the complexity of the resulting TP model. The control structure is a parameter-varying state feedback and observer. The control design specifications include asymptotic stability, robustness against parameter variations influencing the flutter modes and constraint on the control values in order to keep the control signals low. The developed control system is validated by simulation using the high-fidelity, nonlinear model.

Direct power control for grid-connected doubly fed induction generator using disturbance observer based control

Abstract A disturbance observer based control method for a grid-connected doubly fed induction generator is presented in this study. The proposed control method consists of a state-feedback controller and a disturbance observer (DO). The DO is used to compensate for model uncertainties with the aim of removing the steady-state error. The control objective consists of regulating the stator currents instead of the rotor currents in order to achieve direct control of the stator active and reactive powers. Such a control scheme removes the need for an exact knowledge of the machine parameters to achieve accurate control of the stator active and reactive powers. The main advantage of this control method is ensuring a good transient performance as per the controller design specifications, while guaranteeing zero steady-state error. Moreover, the proposed control method was experimentally validated on a small scale DFIG setup.

A New Analytic Method to Tune a Fractional Order PID Controller

This paper proposes a new method to tune a fractional order PID controller. This method utilizes both the analytic and numeric approach to determine the controller parameters. The control design specifications that must be achieved by the control system are gain crossover frequency, phase margin, and peak magnitude at the resonant frequency, where the latter is a new design specification suggested by this paper. These specifications results in three equations in five unknown variables. Assuming that certain relations exist between two variables and discretizing one of them, a performance index can be evaluated and the optimal controller parameters that minimize this performance index are selected. As a case study, a third order linear time invariant system is taken as a process to be controlled and the proposed method is applied to design the controller. The resultant control system exactly fulfills the control design specification, a feature that is laked in numerical design methods. Through matlab simulation, the step response of the closed loop system with the proposed controller and a conventional PID controller demonstrate the performance of the system in terms of time domain transient response specifications (rise time, overshoot, and settling time).

ROBUST FUZZY CONTROL DESIGN USING GENETIC ALGORITHM OPTIMIZATION APPROACH: CASE STUDY OF SPARK IGNITION ENGINE TORQUE CONTROL

In the case of widely-uncertain non-linear system control design, it was very difficult to design a single controller to overcome control design specifications in all of its dynamical characteristics uncertainties. To resolve these problems, a new design method of robust fuzzy control proposed. The solution offered was by creating multiple soft-switching with Takagi-Sugeno fuzzy model for optimal solution control at all operating points that generate uncertainties. Optimal solution control at each operating point was calculated using genetic algorithm. A case study of engine torque control of spark ignition engine model was used to prove this new method of robust fuzzy control design. From the simulation results, it can be concluded that the controller operates very well for a wide uncertainty.

A New Method to Tune a Fractional-Order PID Controller for a Twin Rotor Aerodynamic System

This paper proposes a new method to tune a fractional-order Proportional–Integral–Derivative controller (also called \(\mathrm {P}\mathrm {I}^{\uplambda }\mathrm {D}^{\upmu }\) ) for a Twin Rotor Aerodynamic System, which is a nonlinear highly coupled Multi-Input–Multi-Output system. The five parameters of the controller are tuned using optimization to minimize a performance index which is a weighted sum of absolute values of four frequency domain specifications: gain crossover frequency, phase margin, ISO damping property (to be robust against process gain variation), and magnitude peak value at the resonant frequency, where the latter is the new control design specification that is suggested by this paper. The performance of the proposed controller is compared with that of an integer-order Proportional–Integral–Derivative (PID) controller. Simulation results shows that the fractional-order controller outperforms its integer-order counterpart in minimizing the performance index, which results in satisfying the required design specification more accurately. This is demonstrated by first testing the performance of the of the closed-loop system where the fractional-order controller gives better performance than the integer-order controller and second by testing the robustness of the system by changing one of the process parameters, where the fractional-order controller is much more robust, unlike the integer-order controller where the closed-loop system becomes unstable.

Fractional-order controller design in frequency domain using an improved nonlinear adaptive seeker optimization algorithm

Two nonlinear adaptive versions of the conventional seeker optimization algorithm (SOA) have been proposed for the design of fractional-order controllers using frequency domain specifications. The highly nonlinear and undetermined nature of equations resulting from controller design specifications rules out obtaining a closed-form solution. In this regard, the controller design task has been formulated as an optimization problem and solved using modified variants of the SOA. With the nonlinear adaptation of tuning parameters, the proposed variants of the SOA increase the probability of finding global optima along with improved convergence speed. This has been achieved by incorporating exponential weighting and chaotic behavior in the search process. The validation of the proposed techniques on a set of fractional-order controller design problems clearly exhibits their superiority over other algorithms and controller design techniques. The hardware implementation of the controllers on a DSP TMS320F2812 board ensures applicability for real-time applications.

Retrofit interventions in non-conditioned rooms: calibration of an assessment method on a farm winery

In the Mediterranean area, most wine farms make use of unconditioned above-ground buildings for wine conservation and ageing. Those buildings are usually constructed without a specific temperature control design, therefore indoor temperature distributions and trends can be suboptimal for wine storage. At the same time, various elements (commonly available in farms such as straw bales and climbing plants) could be used to create cheap constructive solutions capable of improving building's thermal behaviour. This study aims to test the effectiveness of different retrofit interventions for thermal behaviour improvement in unconditioned above-ground farm buildings, referring to wine-ageing requirements. The alternative design solutions are evaluated through energy simulations, calibrated and validated on an Italian case study. The results show the importance of roof insulation, and the improved performance of the combination of more retrofit strategies. The proposed approach can be extended to other passive strategies and proves useful to provide information to optimize, or even avoid, conditioning systems.

Observer-Based Stabilizing Controllers for Discrete-Time Systems with Quantized Signal and Multiplicative Random Noise

This paper is concerned with the characterization of quadratic mean square (QMS) stabilizing controllers for systems with both quantization error and multiplicative random noise. An observer-based output feedback controller is proposed and designed to achieve the QMS stability of the closed-loop system, and all QMS stabilizing controllers in this structure are characterized in terms of an inequality involving the Mahler measure of the plant, together with an algebraic Riccati inequality. It is noted that the present results potentially pave the road to search for solutions to a more specific control design to address additional system performance requirements.