Anti-windup Mechanism Research Articles

Sensorless State Control of Stand-Alone Doubly Fed Induction Generator Supplying Nonlinear and Unbalanced Loads

Control methods of a stand-alone doubly fed induction generator mostly use either proportional-integral regulators in multiple synchronous reference frames or proportional-integral-resonant regulators in a stationary reference frame. These control schemes work well when the plant model is not complex and system decoupling is implemented into the control algorithm. However, when the control system contains multiple resonant terms in order to compensate high harmonics of load current, tuning of many regulators in different coordinate systems becomes problematic. The paper presents the application of a linear-quadratic regulator and a rotor position observer in sensorless control of a stand-alone doubly fed induction generator supplying nonlinear and unbalanced loads. Cooperation of the observer and state controller has been validated in the laboratory experiment. An anti-windup mechanism has been implemented into the resonant terms, and it has been shown that the anti-windup structure is crucial for high performance of the state controller with resonant terms. Further, research on observers based on a model reference adaptive system scheme is done in the paper.

Anti-jackknife reverse tracking control of articulated vehicles in the presence of actuator saturation

ABSTRACTIt is well known that backward motion control of an articulated vehicle is difficult because it is an open loop unstable system and such motion is also dangerous due to ‘jackknifing’. In this paper, an anti-jackknife reverse tracking control strategy for autonomous articulated vehicles is proposed based on the combined longitudinal and lateral control scheme. In the proposed lateral-longitudinal control scheme, the major task is to control the reverse heading of the trailer by automatic steering strategies that observe both the anti-jackknife condition and input limitations. The main contribution of this paper is the development of globally asymptotic anti-jackknife stabilising and tracking controls of heading angles with both state and input constraints considered a priori. The proposed control inherently has an anti-windup mechanism that prevents the hitch angle from going beyond any specified critical value to avoid jackknifing, during which time, the steering angle remains at its limit. Stability of the controller is theoretically proven via the Lyapunov argument. Effectiveness of the proposed approach is demonstrated by CarSim and Simulink joint simulations.

Control design methods with anti-windup mechanism for disturbance rejection and reference following

There are many methods for designing feedback control systems with the objectives of disturbance rejection and reference following. Two well-known methods are to directly embed a disturbance model in the controller or to compensate for the effect of disturbance using its estimation from an observer. It is demonstrated in this paper that the first method has inherited a two-degree-of-freedom control system configuration, but has an integrator windup problem in the presence of actuator saturation, whilst the second approach has embedded an anti-windup mechanism, but suffers from poor performance in reference following. This paper examines these two mainstream control design methods for their applications in the presence of actuator saturation. It is shown with a simple example that the classical control system by directly embedding a disturbance model leads to integrator windup, and an anti-windup mechanism is hence proposed to overcome this drawback. The frequency response analysis presented in this paper leads to the conclusion that both methods offer similar closed-loop performance in terms of disturbance rejection and noise attenuation, however, the first approach that embeds a disturbance model in the controller provides a two-degree-of-freedom control system configuration, and hence gives a better performance in terms of reference following. Comparative simulation studies of both Single-Input and Single-Output (SISO) and Multi-input and Multi-Output (MIMO) systems are used to support the conclusions.

Integration of low level controller constraints in supervisory control of buildings

Model Predictive Control (MPC) has been widely used and proved efficient in the control of building installations. It is particularly efficient in the supervisory control of equipment since it can integrate economic, social and environmental dimensions in the computation of the control input. In practice, these equipment possess their own devoted and integrated controllers, which include power saturations and operating modes. Therefore, equipment's inputs cannot be directly controlled by supervisory controllers. This paper aims at analysing to what extent MPC controllers can incorporate existing integrated controllers without introducing extra complexity to their strategies. A Proportional-Integral (PI) controller, with an anti-windup mechanism, has been implemented as the so-called integrated controller. A proposed MPC strategy is discussed. Despite the hybrid functioning of the PI controller, it maintains the simplicity of the problem solving. Furthermore, a comparison between the performances of a benchmark MPC controller (which would drive directly an equipment possessing no integrated controller) and the developed MPC controller is presented. For this study, an office building is targeted and the heating issue has been addressed.

Saturation in Engineering Simulation

The wind-up effect is a phenomenon known mostly from implementing PI(D) controllers in real operation when saturation of the manipulated variable may occur and cause worse control results. Nowadays many publications exist on this topic where several anti wind-up mechanisms are presented and discussed. However, the wind-up phenomenon, i.e. the problem of continued integration after the output has become limited (saturated), is a problem appearing not only in connection with numerically calculated control actions of digital controllers, but also in computer models of controlled plants. In this field, existence of the wind-up effect is somehow missed. Results and conclusions obtained by such inaccurate simulation may be misleading. Surprisingly, occurrence of this situation is not so rare, because use of amplitude limiting blocks in simulation is considered to be the only necessary precaution. In education of many control engineers, information about wind-up effect in simulation is not sufficiently pointed out. In models of higher than first order, the same approach as the one used the filed of PI(D) controllers is not always correct. The publications focused on the solutions for this case seem to be rare, if any exist. This paper presents one possible approach to wind-up and anti wind-up mechanism in modeling and simulations.

Actuator saturation and anti-windup compensation in event-triggered control

Event-triggered control aims at reducing the communication load over the feedback link in networked control systems by sending information only if certain event conditions, which guarantee a desired control performance, are satisfied. This article investigates the consequences of actuator saturation on the behavior of the event-triggered control loop in terms of its stability and information exchange. Stability properties are derived using linear matrix inequalities (LMIs) which show how the stability of the event-triggered control loop depends on the selection of the event threshold. Moreover, it is shown that a lower bound on the minimum inter-event time exists being likewise affected by the event threshold. As actuator saturation might severely degrade the performance of the event-triggered closed-loop system, the scheme is extended by incorporating an anti-windup mechanism in order to overcome this problem. The results are illustrated by simulations and experiments.

Automatic dosage of hydrogen peroxide in solar photo-Fenton plants: Development of a control strategy for efficiency enhancement

The solar photo-Fenton process is widely used for the elimination of pollutants in aqueous effluent and, as such, is amply cited in the literature. In this process, hydrogen peroxide represents the highest operational cost. Up until now, manual dosing of H2O2 has led to low process performance. Consequently, there is a need to automate the hydrogen peroxide dosage for use in industrial applications. As it has been demonstrated that a relationship exists between dissolved oxygen (DO) concentration and hydrogen peroxide consumption, DO can be used as a variable in optimising the hydrogen peroxide dosage. For this purpose, a model was experimentally obtained linking the dynamic behaviour of DO to hydrogen peroxide consumption. Following this, a control system was developed based on this model. This control system – a proportional and integral controller (PI) with an anti-windup mechanism – has been tested experimentally. The assays were carried out in a pilot plant under sunlight conditions and with paracetamol used as the model pollutant. In comparison with non-assisted addition methods (a sole initial or continuous addition), a decrease of 50% in hydrogen peroxide consumption was achieved when the automatic controller was used, driving an economic saving and an improvement in process efficiency.

On Immediate, Delayed and Anticipatory Activation of Anti-Windup Mechanism: Static Anti-Windup Case

This technical note examines the immediate, delayed and anticipatory activation of anti-windup mechanism. Traditional anti-windup compensators are designed for immediate activation. A recent innovation is to delay the activation of the anti-windup mechanism until the degree of saturation reaches to a certain level. By allowing the nominal controller to function undisturbed in the face of modest actuator saturation, delayed activation of the anti-windup mechanism has been shown to lead to improvement in the closed-loop performance. In this technical note, we propose to activate the static anti-windup compensation in anticipation of actuator saturation and show that such a “precautionary” approach has the potential of leading to further improvement in the closed-loop performance, in terms of both the transient quality in reference tracking and the region of stability.

Antiwindup-aware PI autotuning

This manuscript addresses the problem of “tuning the antiwindup mechanism” of PI(D) controllers, an issue seldom addressed in the literature but yielding improvement and reducing undesired large-transient behaviours if tackled correctly. A previously proposed framework is extended and related to both alternative antiwindup implementations and tuning methods, resulting in more methodological insight on the matter, and guidelines to set up the extension of a given method to include antiwindup. Simulation examples are reported to illustrate the achievable advantages.

A feedback-based [formula omitted]-norm limiter for periodic signals

Limiting the rms value ( L 2 norm) of periodic signals can be a necessity in some controllers for power electronic devices. The prevailing existing solution is simply to compute, off-line, the L 2 norm and then to multiply the signal by the factor that ensures the desired bound is achieved. This feedforward procedure could yield unsatisfactory results during the transients of the system. In this paper, a feedback-based algorithm that carries out the computations on-line is proposed. Besides the on-line computation of the L 2 norm, the second component of the algorithm is a classical proportional–integral loop with an anti-windup mechanism. Although the overall algorithm is described by complicated nonlinear, non-autonomous, delay-differential equations, a detailed stability analysis is carried out, proving its robust performance. Simulation results, that confirm the theoretical results, are presented.

Generalized sector synthesis of output feedback control with anti-windup structure

This paper considers the control synthesis problem for linear time invariant (LTI) continuous-time systems with actuator saturation nonlinearities. The control architecture is standard; a nominal LTI output feedback with an anti-windup (AW) mechanism. Traditional approaches have focused on AW synthesis where the nominal controller is first designed to achieve a local performance in the linear region of the actuators and then the AW compensator is added to guarantee stability/performance properties when the actuators saturate. In contrast, this paper proposes a synthesis method to design both the nominal controller and the AW compensator simultaneously. Using the generalized sector (GS) approach in the multiplier setting, a sufficient condition is given in terms of linear matrix inequalities so that a given ellipsoid is synthesized as a domain of attraction for the closed-loop system. Our result is shown to include the existing results on the simultaneous circle synthesis and the anti-windup GS synthesis as special cases. This paper also shows that the direct feed-through term in the AW compensator does not contribute to enlarging the achievable domain of attraction within the multiplier GS framework.

NONLINEAR GENERALIZED MINIMUM VARIANCE CONTROL UNDER ACTUATOR SATURATION

A new Generalized Minimum Variance control law has been derived recently for the control of nonlinear multivariable systems. In this paper we restrict our interest to single-input, single-output plants with input nonlinearities in the form of hard actuator limits. Since in real systems saturation always exists in some form, e.g. as a result of valve opening limits or finite power supply, this is a natural case to consider. One of the well-known problems associated with input saturation is the integral windup phenomenon, which occurs whenever the controller includes integral action. In this paper, we show that the classical form of the “anti-windup” mechanism can be obtained within the Nonlinear GMV controller framework by a suitable selection of the design parameters. The advantage of the approach is that the anti-windup mechanism is obtained naturally from the optimization problem. There is also the possibility that the technique can be extended for other specialized nonlinear compensation problems.

Anti-Windup Implementation in a Fuzzy-Logic Model-Following Robust PID Controller

The paper deals with dynamical saturation occurring in classic and fuzzy logic based PID algorithms, and in the corrective loop of the model following control structure (MFC). To cope with the problem a modified anti-windup mechanism has been proposed. This approach offers a substantial increase in control performance when the reference signal is close to its maximal or minimal value, and also removes overshoots experienced by the controlled signal.

MCMAC–CVT: a novel on-line associative memory based CVT transmission control system

This paper describes a novel application of an associative memory called the Modified Cerebellar Articulation Controller (MCMAC) (Int. J. Artif. Intell. Engng, 10 (1996) 135) in a continuous variable transmission (CVT) control system. It allows the on-line tuning of the associative memory and produces an effective gain-schedule for the automatic selection of the CVT gear ratio. Various control algorithms are investigated to control the CVT gear ratio to maintain the engine speed within a narrow range of efficient operating speed independently of the vehicle velocity. Extensive simulation results are presented to evaluate the control performance of a direct digital PID control algorithm with auto-tuning (Trans. ASME, 64 (1942)) and anti-windup mechanism. In particular, these results are contrasted against the control performance produced using the MCMAC (Int. J. Artif. Intell. Engng, 10 (1996) 135) with momentum, neighborhood learning and Averaged Trapezoidal Output (MCMAC–ATO) as the neural control algorithm for controlling the CVT. Simulation results are presented that show the reduced control fluctuations and improved learning capability of the MCMAC–ATO without incurring greater memory requirement. In particular, MCMAC–ATO is able to learn and control the CVT simultaneously while still maintaining acceptable control performance.

Temperature control of a solar furnace

An application of an automatic control strategy to a solar plant for material treatment has been shown. Since these kinds of plants are manually operated by skilled operators, the development of an automatic control strategy aimed at achieving adequate results throughout the wide range of operating conditions, under which these plants operate, represents an important improvement towards facilitating operation and obtaining desired performance. The control scheme presented is based on a PI controller which incorporates feedforward compensation, an anti-windup mechanism and actuator slew rate constraint handling, both in fixed and self-tuning configurations. It has been developed and applied to the control of a solar furnace for samples of different materials under extreme temperature profiles. Different plant results have been shown and both advantages and drawbacks of the scheme have been commented on.