Control Input Saturation Research Articles

Increasing the operating area of shunt active filters by advanced nonlinear control

This work is aimed to rigorously manage voltage saturation and maximum current constraints in Shunt Active Filters. In thisrespect, assuming “unconstrained” control algorithms have already been defined to achieve standard objectives for such devices(i.e. current tracking for harmonic compensation and DC-bus voltage boundness), a plug-in unit, oriented to extend the systemoperating region and at the same time preserving good performance under large transients and overload conditions, is presented.This solution allows to improve availability, robustness and composability of Shunt Active Filters, which are expected to be keyfeatures in present and next generation complex and possibly “smart” power grids. The proposed unit is composed by two parts.First, a suitable anti-windup strategy is defined in order to deal with control input saturation. Its main purpose is to preserve theoriginal “unconstrained error dynamics”, in face of input saturation, while guaranteeing low computational burden and reducedperformance impairment (the latter goal, in harmonic compensation context, leads to rather non-standard problem formulation).To this aim, the anti-windup acts on the current references through a suitably-designed additional dynamics. Then, in order to copewith current limitations, an additional strategy has been designed; again the current references is suitably shaped to comply withthe features and bounds of the system, augmented with the above-mentioned anti-windup solution. The proposed scheme can besimply joined to any kind of unconstrained controller adopted to steer Shunt Active Filters. In this work, an Internal-Model-basedcurrent controller is adopted as a benchmark case. The proposed approach is validate through extensive simulation tests.

Disturbance observer-based robust guidance for Mars atmospheric entry with input saturation

With low-lifting capability taken into account, a robust guidance law for Mars entry vehicles with low lift-to-drag ratios, such as Mars Science Laboratory (MSL), is presented. Consider the nonlinear term in the drag dynamic equation and bounded disturbances as a lumped disturbance, and design a linear disturbance observer (DOB) to estimate it. With the consideration of the control input saturation, an innovative sliding surface and a virtual system are introduced to design the guidance law. Analyses of disturbance observer performance and Lyapunov-based transient performance are also presented. It is shown that the drag tracking error can be adjustable by explicit choices of design parameters. Simulation results confirm the effectiveness of the proposed guidance law.

An online reference governor for mobile robot to reduce the occurrence of control input saturation

This study proposed an online reference governor for a mobile robot to reduce the occurrence of control input saturation. For following the trajectory by a mobile robot, it is one of the practical subjects to provide appropriate control reference even if any disturbances occur. We proposed a methodology to regulate the control reference iteratively based on time-scaling approach. The time-scaling approach is a method to realize to regulate time development characteristic on the given trajectory. It is difficult to model the effect of the interaction with the road surface and the trajectory tracking error is appeared as the amount of accumulated such factors. Therefore, it is a practical approach to reduce the occurrence of control input saturation based on the evaluation of the trajectory tracking error. Proposed reference governor realizes online time scaling based on the trajectory tracking error index and a smooth transition dynamics. By introducing the proposed method, the occurrence of control input saturation can be reduced in case of that the disturbances occur. For verification of our proposed method, computer simulations utilizing a stable velocity controller were conducted and the results were discussed.

Integrated guidance and control law for cooperative attack of multiple missiles

An integrated guidance and control law is developed, using the dynamic surface control theory and disturbance observation technology, for multiple missiles attacking targets cooperatively. The modeling error, aerodynamic parameters perturbation and external disturbance are regarded as system uncertainties, and then integrated guidance and control model of missile is built. Considering multiple missiles' cooperation of flight position and impact time, the constraint of maximum target look angle and the requirement of trajectory convergence, a cooperative strategy expressed by desired target look angle command is proposed. To analyze the effect of nonlinear saturation of missile control input on integrated guidance and control system, a supplementary subsystem is introduced. Three disturbance observers are designed to estimate the system uncertainties. With the signals generated by supplementary subsystem and uncertainty estimate from disturbance observers, a robust controller is designed using dynamic surface control theory to track the desired target look angle command. Based on Lyapunov stability theory and comparison principle, the stability of system under the condition of unknown uncertainties and control input saturation is proved, and the transient tracking error is also discussed. Simulation results demonstrate the effectiveness and robustness of the integrated guidance and control law.

Control of Shunt Active Filters With Actuation and Current Limits

In this brief, a plug-in unit is presented to manage control voltage saturation and maximum current limit in shunt active filters (SAFs), where unconstrained control algorithms are already defined. The proposed unit extends the operating region of such devices, i.e., under large transients and overload conditions, with performance guarantees. Therefore, improved robustness, availability, and composability of SAFs are obtained. The solution is composed of two parts. An antiwindup (AW) unit is defined to deal with control input saturation by modifying the current references through a suitably designed additional dynamics. In addition, a current saturation strategy is formulated. Again the current reference is modified, accounting for the limitations of the system, augmented with the AW scheme. The approach is valid for any kind of unconstrained controller adopted to steer SAFs. Here, results are presented considering an internal-model-based current controller. Simulation and experimental tests confirm the effectiveness of the method.

Robust Model Predictive Control Based on Nominal System Optimization and Control Input Saturation

The paper presents in detail alternative robust model predictive control approach based on the optimization of convergence rate subject to nominal system, and additional saturation of control inputs. The approach is a compromise between guaranteed convergence rate and high computational complexity on the one hand, and larger set of feasible initial conditions and lower computational burden on the other hand. The applicability of the proposed strategy is verified using a case study of uncertain chemical reactor stabilization.

Synchronization of dynamic network subjected to control input saturation

This paper addresses the issue of dynamic networks synchronization. Synthesized decentralized control algorithm provides the required accuracy in tracking of the leading subsystem under conditions of the parametric uncertainties, external disturbances and saturation of the control channels. Computer simulations illustrate the efficiency of the algorithm.

Saturated Adaptive Output-Feedback Power-Level Control for Modular High Temperature Gas-Cooled Reactors

Small modular reactors (SMRs) are those nuclear fission reactors with electrical output powers of less than 300 MWe. Due to its inherent safety features, the modular high temperature gas-cooled reactor (MHTGR) has been seen as one of the best candidates for building SMR-based nuclear plants with high safety-level and economical competitive power. Power-level control is crucial in providing grid-appropriation for all types of SMRs. Usually, there exists nonlinearity, parameter uncertainty and control input saturation in the SMR-based plant dynamics. Motivated by this, a novel saturated adaptive output-feedback power-level control of the MHTGR is proposed in this paper. This newly-built control law has the virtues of having relatively neat form, of being strong adaptive to parameter uncertainty and of being able to compensate control input saturation, which are given by constructing Lyapunov functions based upon the shifted-ectropies of neutron kinetics and reactor thermal-hydraulics, giving an online tuning algorithm for the controller parameters and proposing a control input saturation compensator respectively. It is proved theoretically that input-to-state stability (ISS) can be guaranteed for the corresponding closed-loop system. In order to verify the theoretical results, this new control strategy is then applied to the large-range power maneuvering control for the MHTGR of the HTR-PM plant. Numerical simulation results show not only the relationship between regulating performance and control input saturation bound but also the feasibility of applying this saturated adaptive control law practically.

Lyapunov‐based robust control design for a class of switching non‐linear systems subject to input saturation: application to engine control

Control technique based on the well-known Takagi–Sugeno (T–S) models offers a powerful and systematic tool to cope with complex non-linear systems. This study presents a new method to design robust H ∞ controllers stabilising the switching uncertain and disturbed T–S systems subject to control input saturation. To this end, the input saturation is taken into account in the control design under its polytopic form. The Lyapunov stability theory is used to derive the design conditions, which are formulated as a linear matrix inequality (LMI) optimisation problem. The controller design amounts to solving a set of LMI conditions with some numerical tools. In comparison with previous results, the proposed method not only provides a simple and efficient design procedure to deal with a large class of input saturated non-linear systems but also leads to less conservative design conditions. Moreover, with a simple shape criterion, the proposed approach maximises also the estimated domain of attraction included inside the validity domain of the system. The validity of the proposed method is illustrated through academic as well as real industrial examples.

Decentralized sliding-mode control for spacecraft attitude synchronization under actuator failures

This paper examines attitude synchronization and tracking problems with model uncertainties, external disturbances, actuator failures and control torque saturation. Two decentralized sliding mode control laws are proposed and analyzed based on algebraic graph theory. Using Barbalat׳s Lemma, it is shown that the control laws guarantee each spacecraft approaches the desired time-varying attitude and angular velocity while maintaining attitude synchronization among the other spacecraft in the formation. The first controller is designed in the presence of model uncertainties, external disturbances, and actuator failures. The results are extended to the case with control input saturation in the second controller. Both control laws do not require online identification of failures. Numerical simulations are presented to show the effectiveness of the proposed attitude synchronization and tracking approaches.

A robust composite nonlinear control scheme for servomotor speed regulation

A parameterised design of robust composite nonlinear controller is proposed for typical second-order servo systems subject to unknown constant disturbance and control input saturation. The control law consists of a linear feedback part for achieving fast response, a nonlinear feedback part for suppressing the overshoot, and a disturbance-compensation mechanism for erasing the steady-state error. An extended state observer is adopted to estimate the unknown disturbance. The closed-loop stability is analysed theoretically. The control scheme is applied to the speed regulation of permanent magnet synchronous motor, and numerical simulations are carried out. The results confirm that the proposed control scheme can achieve fast, smooth, and accurate speed regulation, and has a certain degree of robustness with respect to the amplitude of disturbances and the perturbations of system parameters.

Adaptive Fault-Tolerant Spacecraft Attitude Control Design With Transient Response Control

This paper proposes an adaptive fault tolerant attitude controller, based on variable structure control, to address tracking control problem of spacecraft in the presence of unknown actuator failure, control input saturation and external disturbances. In contrast to traditional robust fault tolerant attitude controllers, the proposed controller is capable of controlling the transient response of the closed loop system by means of a dedicated parameter. Also this method does not require exact knowledge of the actuator faults and is implemented with uncertain value of fault information. By adjusting a simple parameter dynamically and using Lyapunov direct method and the properties of quaternion representation, the ultimate boundedness of attitude and angular velocity errors in presence of external disturbances is proven. Numerical simulations used to prove the success of proposed controller in achieving high attitude performance in the presence of external disturbances, inexact knowledge of fault information, actuator failures and control input saturation.

Robust bounded control for uncertain flight dynamics using disturbance observer

The robust bounded flight control scheme is developed for the uncertain longitudinal flight dynamics of the fighter with control input saturation invoking the backstepping technique. To enhance the disturbance rejection ability of the robust flight control for fighters, the sliding mode disturbance observer is designed to estimate the compounded disturbance including the unknown external disturbance and the effect of the control input saturation. Based on the backstepping technique and the compounded disturbance estimated output, the robust bounded flight control scheme is proposed for the fighter with the unknown external disturbance and the control input saturation. The closed-loop system stability under the developed robust bounded flight control scheme is rigorously proved using the Lyapunov method and the uniformly asymptotical convergences of all closed-loop signals are guaranteed. Finally, simulation results are presented to show the effectiveness of the proposed robust bounded flight control scheme for the uncertain longitudinal flight dynamics of the fighter.

Adaptive dynamic surface control of a class of nonlinear systems with unknown direction control gains and input saturation.

In this paper, adaptive neural network based dynamic surface control (DSC) is developed for a class of nonlinear strict-feedback systems with unknown direction control gains and input saturation. A Gaussian error function based saturation model is employed such that the backstepping technique can be used in the control design. The explosion of complexity in traditional backstepping design is avoided by utilizing DSC. Based on backstepping combined with DSC, adaptive radial basis function neural network control is developed to guarantee that all the signals in the closed-loop system are globally bounded, and the tracking error converges to a small neighborhood of origin by appropriately choosing design parameters. Simulation results demonstrate the effectiveness of the proposed approach and the good performance is guaranteed even though both the saturation constraints and the wrong control direction are occurred.

Fuzzy control for Electric Power Steering System with assist motor current input constraints

Friction and disturbances of the road are the main sources of nonlinearity in the Electric Power Steering (EPS) System. Consequently, conventional linear controllers design based on a simplified linear model of the EPS system will result in poor dynamic performance or system instability. On the other hand, a brush-type DC motor is more used in EPS control with an input current that is limited in practice. The control laws designed without taking into account the saturation effect may have undesirable consequences on the system stability. In this paper, a Takagi–Sugeno (T−S) fuzzy is used to represent the nonlinear behavior of an EPS system, and stabilization conditions for nonlinear EPS system with both constrained and saturated control input cases are proposed in terms of linear matrix inequalities (LMI). Simulation results show that both the saturated and constrained controls can stabilize the resulting closed-loop EPS system and provide a stable driving in the presence of nonlinear friction, disturbance of the road and actuator saturation.

Iterative learning and adaptive fault‐tolerant control with application to high‐speed trains under unknown speed delays and control input saturations

This study investigates the speed trajectory tracking problem of high-speed trains with actuator failures and unknown speed delays as well as control input saturations. New adaptive iterative learning fault-tolerant control (AILFTC) strategy is derived without the need for precise system parameters or analytically estimating bound on actuator failures variables. It is shown that with the proposed method, both actuator failures can be accommodated and the unknown time-varying speed delays and control input saturations can be analysed by means of Lyapunov–Krasovskii function. As such, the resultant control algorithms are able to achieve the L [0,T] 2 convergence of the train speed to desired profile during operations repeatedly in the presence of non-linearities and parametric uncertainties, as validated by the theoretical analysis and numerical simulations.

Decentralized time-varying formation control for multi-robot systems

In this paper, a distributed controller–observer schema for tracking control of the centroid and of the relative formation of a multi-robot system with first-order dynamics is presented. Each robot of the team uses a distributed observer to estimate the overall system state and a motion control strategy for tracking control of time-varying centroid and formation. Proof of the overall convergence of the controller–observer schema for different kinds of connection topologies, as well as for the cases of unsaturated and saturated control inputs is presented. In particular, the solution is proven to work in the case of strongly connected non-switching topologies and in the case of balanced strongly connected switching topologies. In order to complete the work, the approach is validated by experimental tests with a team of five wheeled mobile robots.

Robust Saturated Finite Time Output Feedback Attitude Stabilization for Rigid Spacecraft

This paper investigates the velocity-free feedback control problem associated with finite time attitude stabilization of a rigid spacecraft subject to external disturbance and input saturation. First of all, to address the lack of angular velocity measurement, a novel, fast, finite time convergent observer is proposed to recover the unknown angular velocity information in a finite time under external disturbance. Then, a finite time output feedback controller is proposed using the designed finite time observer, in which the control input saturation nonlinearity is explicitly considered in the proposed finite time stabilizer. Rigorous proofs show that the proposed observer can achieve finite time stability, the controller rigorously enforces actuator-magnitude constraints, and the attitude of the rigid spacecraft will converge to the equilibrium in a finite time. Numerical simulations illustrate the spacecraft performance obtained using the proposed controllers.

Robust Control of Automotive Active Seat-Suspension System Subject to Actuator Saturation

This paper deals with the problem of robust sampled-data control for an automotive seat-suspension system subject to control input saturation. By using the nature of the sector nonlinearity, a sampled-data based control input saturation in the control design is studied. A passenger dynamic behavior is considered in the modeling of seat-suspension system, which makes the model more precisely and brings about uncertainties as well in the developed model. Robust output feedback control strategy is adopted since some state variables, such as, body acceleration and body deflection, are unavailable. The desired controller can be achieved by solving the corresponding linear matrix inequalities (LMIs). Finally, a design example has been given to demonstrate the effectiveness and advantages of the proposed controller design approach.

Global tracking for underactuated ships with bounded feedback controllers

In this paper, we present a global state feedback tracking controller for underactuated surface marine vessels. This controller is based on saturated control inputs and, under an assumption on the reference trajectory, the closed-loop system is globally asymptotically stable. It has been designed using a 3 degrees of freedom benchmark vessel model used in marine engineering. The main feature of our controller is the boundedness of the control inputs, which is an essential consideration in real life. In absence of velocity measurements, the controller works and remains stable with observers and can be used as an output feedback controller. Simulation results demonstrate the effectiveness of this method.