Uncertainty And Disturbance Estimator Research Articles

Spacecraft Medium Voltage Direct-Current (MVDC) Power and Propulsion System

This paper introduces a medium voltage direct-current (MVDC) system for large spacecraft megawatt-scale (MW) power and propulsion systems intended for interplanetary transport, including missions to the Moon and Mars. The proposed MVDC system includes: (i) A nuclear electric propulsion (NEP) that powers a permanent magnet (PM) generator whose output is rectified and connected to the MVDC bus. (ii) A solar photovoltaic (PV) source that is interfaced to the MVDC bus using a unidirectional boost DC-DC converter. (iii) A backup battery energy storage system (BESS) that connects to the MVDC bus using a bidirectional DC-DC boost converter. (iv) A dual active bridge (DAB) converter that controls the power to the spacecraft’s electric thruster. The NEP serves as the main power source for the spacecraft’s electric thruster, while the solar PV and BESS are intended to provide power for the payload and spacecraft’s low-voltage power system. The paper will (i) provide a review of the spacecraft MVDC power and prolusion system highlighting state-of-the-art main components, (ii) address the control of boost converters for the PV and BESS sources and the DAB converter for the thruster, and (iii) propose an uncertainty and disturbance estimator (UDE) concept based on current control algorithms to mitigate MVDC instability due to unpredictable factors and external disruptions. The proposed UDE can actively estimate and compensate for the system disturbance and uncertainty in real time, and thus, both the system tracking performance and robustness can be improved. Simulation studies have been conducted to substantiate the efficacy of the proposed schemes.

Intelligent robust control for aviation electric fuel pump based on second order integral sliding mode with UDE

This paper proposes an original intelligent robust control method for a kind of aviation electric fuel pump (AEFP) systems, in order to realize on-demand fuel supply for aircraft engines, accurately and robustly. By considering the feature of AEFP, the intelligent robust control method is in a cascaded structure, which combines a novel second order integral sliding mode control (SOISMC) approach with uncertainty and disturbance estimator (UDE) strategy, together with a new multi-objective variable speed grey wolf optimization algorithm (MOVSGWO). To let the current loop of AEFP possess both rapidity and robustness, the SOISMC is presented based on an innovative second order integral sliding mode manifold. To realize that AEFP has strong robustness to both the changing of speed command and the pulsation in fuel pump, UDE is utilized for the speed loop of AEFP. The primary parameters in the intelligent robust controller, namely, UDE-SOISMC controller, are optimized by MOVSGWO. The MOVSGWO is a modified grey wolf optimization algorithm with variable speed by particle swarm optimization. Mismatched uncertainties and disturbances in AEFP can be handled by the proposed robust method. The calculation of instantaneous flow and dynamic torque in AEFP is illustrated. Simulation and experimental results verify the effectiveness of the UDE-SOISMC method.

Continuous finite-time terminal sliding mode to solve the tracking problem in a class of mechanical systems

The major contribution of this study is the feedback design of a finite-time convergence sliding mode control to solve the trajectory-tracking problem in a class of mechanical systems. Some advantages are that the controller presents a continuous signal by integration of the high-frequency switching term. Another benefit is the design and implementation of an uncertainty and disturbance estimator (UDE) to robustify the closed-loop system. We use Lyapunov tools to develop the closed-loop stability analysis and to give an expression of the convergence time [Formula: see text] t through this, we can reduce the convergence time by tuning the gains of the controller. We illustrate the performance of the proposed control structure via numerical simulations conducted for a mass-spring-damper system and experiments developed in a pendular system.

Anti-Windup Pitch Angle Control for Wind Turbines Based on Bounded Uncertainty and Disturbance Estimator

Due to physical limitations and safety requirements, the rate and amplitude of change in wind turbines’ pitch angle are limited, which will bring integral saturation problems to the control system. This leads to the deterioration of the pitch control system’s performance or even an instability problem. This paper designs an anti-windup robust pitch angle control strategy to deal with pitch rate constraint issue to enhance the safety of the control system. First, to facilitate controller design, a filtered tracking-error technique is employed to transform the nonaffine form into an affine one. Subsequently, a feedback robust controller based on an uncertainty and disturbance estimator (UDE) is developed to handle the model’s uncertainty and external disturbances. To address the issue of integral saturation in the pitch system and guarantee its safety, an elliptical bounded constraint is integrated into the designed UDE strategy. This bounded UDE controller can improve the stability of power generation quality, reducing the mechanical loads on components. Finally, the effectiveness of the proposed scheme is verified on the Wind Turbine Blockset platform in Matlab/Simulink. It can achieve better performance than traditional methods.

UDE-Based Distributed Formation Control for MSVs With Collision Avoidance and Connectivity Preservation

Limited computational resource is one of features of the embedded system including unmanned marine surface vehicle (MSV). Under this practical constraint, we aim to develop a distributed formation control algorithm for a group of uncertain MSVs, whose computational burden is low. The uncertainty and disturbance estimator (UDE) is employed to compensate for the modeling uncertainties, the unknown environmental disturbances, and the unavailable derivative of the virtual control inputs, such that the formation errors converge asymptotically to zero. Meanwhile, the prescribed performance control technique is applied to ensure that the convergence rates of the formation errors are faster than predefined values. Additionally, collision avoidance and connectivity preservation among the neighboring vehicles are guaranteed, while the obstacle avoidance is also ensured. As no parameter update law is required to calculate, the computational burden is reduced significantly. The effectiveness of the proposed UDE-based formation control algorithm is validated through comparative simulation.

An improved uncertainty and disturbance estimator-based speed control for grid-connected pumping kite wind generator

Pumping kite generator systems (KGS) represent a novel and exciting concept for future wind energy harvesting at higher altitudes where the wind is more reliable, stable, and highly predictable. The configuration of such systems generally employs a flying tethered kite attached to a generator/motor and connected to the grid using a power electronic interface. This work aims to enhance control system efficiency and increase the generated power by improving the speed control of the generator. To this end, an uncertainty and disturbance estimator (UDE) based speed controller is developed to deal with the challenges presented by this new technology, particularly periodical variation of the rotational speed reference in maximum power point tracking. The practicality and effectiveness of the proposed UDE-based control method are demonstrated through simulation and experimental results. It is shown that the proposed control strategy offers improved tracking performance and robustness against model uncertainties and external disturbances.

Time-varying uncertainty and disturbance estimator without velocity measurements: Design and application

The uncertainty and disturbance estimator (UDE) technique has been proven to be simple but effective in reconstructing unknown disturbances. However, classic UDE with a constant gain cannot deal with more complicated issues, such as the trade-off between steady-state and transient performance of the estimator, simultaneous attenuation of input disturbances and measurement noises, etc. Motivated by this observation, the design of UDE technique is extended to the time-varying situation and an implementable form of time-varying UDE (TV-UDE) is proposed. In particular, velocity measurements are assumed to be unavailable in the design. A time-varying ordinary differential equation is constructed to build the estimation relationship. A formula for the integration by parts is thereafter introduced to derive an implementable form of the TV-UDE. The classic UDE is shown to be a special case of the TV-UDE. The UDE is combined with a nominal dynamic output-feedback controller to yield a robust control solution to a class of uncertain second-order attitude control systems without velocity measurements. The design of a TV-UDE is discussed with a low initial gain and a higher final gain to avoid transient performance issues. Finally, numerical simulations and physical experiments are carried out on a 2-DOF AERO attitude helicopter platform to demonstrate the control performance of the solution. It is illustrated that the proposed design can effectively reduce the steady-state errors in both the trajectory tracking and target pointing tasks. Meanwhile, the input saturation and peaking phenomena associated with a high-gain estimation are avoided by applying the TV-UDE.

Robust consensus tracking control for switched multiple Lagrangian systems by UDE-based control method

The robust consensus tracking control of the switched multiple Lagrangian systems is investigated by the uncertainty and disturbance estimator (UDE)-based control method. Firstly, the switching control method based on the average dwell time (ADT) algorithm is utilized for parameters and network communication topology switching of multiple Lagrangian systems to reduce the impact of load variation and enhance robustness. Then, in two parts, a proposed adaptive UDE-based control method for multiple Lagrangian systems with lumped uncertainty and load variation. In the initial stage, a consensus tracking control of the nominal switched multiple Lagrangian systems is realized via an adaptive backstepping tracker. The UDE control method is integrated and used in the second stage for the exact adjustment of model uncertainties and disturbances as well as asymptotic estimates. It is worth noting that by designing different filters for UDE, it is possible to handle several specified external disturbances that are unbounded. The unified analysis framework of the closed-loop switched control system is realized through Lyapunov analysis. Therefore, the proposed consensus tracking algorithm has a simple structure and good robustness in handling load changes, model uncertainties, and external disturbances. Finally, illustrative examples with computer simulation are provided to verify the effectiveness and correctness of the obtained results.

Design of UDE-based finite-time fault-tolerant control for DP vessels with complex disturbances and input constraints

This paper presents a novel uncertainty and disturbance estimator (UDE) – based finite-time fault-tolerant control scheme, applied to the controller design of dynamic positioning (DP) vessels with environmental disturbances, model uncertainty, input saturation, and thruster failures. The proposed method employs a non-singular terminal sliding surface to achieve finite-time stability based on the vessel's position and velocity errors. A UDE is to estimate environmental disturbances and model uncertainties. The auxiliary dynamic system (ADS) and smooth switching function are adopted to compensate for input constraints and avoid singularity caused by ADS. Further, a proposed fault observer is to estimate the thruster's fault state. In partial thruster failure, the utilization of the remaining execution ability of the failed thrusters to achieve vessel control objectives. Theoretical and simulation results show that the proposed method has a fast response, high control accuracy, strong disturbance resistance, and fault tolerance, making it superior to recent and classical trajectory tracking control.

TV‐UDE: Time‐varying uncertainty and disturbance estimator

AbstractThe UDE (uncertainty and disturbance estimator)‐based control approach has received increasing attention due to its simplicity and effectiveness in rejecting input disturbances. Despite this fact, the inherent transient performance issues associated with a high‐gain UDE are rarely considered. We first design a classic UDE‐based robust controller for a linear system subject to three types of model uncertainties: exogenous disturbance, state‐dependent uncertainty, and input‐dependent uncertainty. We prove by the singular‐perturbation theory that arbitrarily small steady‐state tracking errors can be achieved by choosing an enough‐high gain for the UDE. We then design a novel time‐varying UDE (TV‐UDE) to improve the transient performance. The key of the improved design is to use a differential equation with a time‐varying parameter, instead of a transfer function, to describe the underlying filtering relationship. The formula for the integration by parts is introduced to derive an explicit computable expression of the TV‐UDE. Interestingly, the TV‐UDE is reduced to a classic UDE if the design parameter is fixed. In addition, two types of smooth and bounded functions are proposed to guide the change of UDE parameter from a large value to a smaller value (i.e., from a small gain to a higher one), to ensure that the UDE generates a smooth high‐accuracy estimate. The advantages of the TV‐UDE are demonstrated by simulation and experimental comparisons on a 2‐DOF AERO attitude control platform. The transient performance issues (such as the peaking phenomenon and obvious oscillation) of high‐gain UDE are effectively avoided.

UDE-Based Dynamic Surface Control for Quadrotor Drone Attitude Tracking under Non-Ideal Actuators

Quadrotor drone attitude tracking is inevitably affected by the combination of model uncertainties, external disturbances, and non-ideal actuator dynamics during stable flight and complex maneuvers. In order to achieve precise attitude control in these situations, a cascade-structured dynamic surface control (DSC) strategy is proposed based on an uncertainty and disturbance estimator (UDE), considering the actuator dynamics as represented by a first-order plus time-delay model. The DSC scheme is used to transform the original attitude dynamics system into a set of interconnected subsystems. On the one hand, the mismatched disturbances in the attitude kinematics and dynamics loops are converted into matched disturbances to accommodate the structural constraints of the UDE so that these disturbances, as well as the non-ideality caused by the actuator time delay, are estimated and compensated for by the approach. On the other hand, the “complexity explosion” problem is addressed by the first-order filter employed by DSC. The ultimate boundedness of the closed-loop system is proven while the parameter design constraints are provided. MATLAB Simulink simulations are conducted to demonstrate the desirability of considering actuator dynamics and to verify that the proposed control strategy can relax the constraints of the control parameters and enable a higher accuracy.

Robust Cooperative Control of UAV Swarms for Dual-Camp Divergent Tracking of a Heterogeneous Target

Agents are used to exhibit swarm intelligence in the sense of convergence, while divergence is equivalently common in nature and useful in complex applications for multi-UAV systems. This paper proposes a robust target-tracking control algorithm, where UAV swarms are partitioned by a signed graph to perform opposite movements along or against the trajectory of the target. Uncertainties take place in both the fractional-order model of the target and the double-integrator dynamics of the UAVs. To tackle the challenge induced by the bipartite behavior and unknown components in the multi-UAV systems, the article comes up with a backstepping cascade controller and a new method for uncertainty estimation-compensation via a combined approach based on a neural network (NN) and an Uncertainty and Disturbance Estimator (UDE). Steered by the controller, UAVs in a structurally balanced network will display symmetry of their paths, pursuing or away from the target with respect to the origin. Theoretical derivation and numerical simulations have evidenced that the tracking errors converge to zero. Compared with the traditional NN method to solve such problems, this method is proposed for the first time, which can effectively improve the precision of cooperative target tracking and reduce the chattering phenomena of the controller.

Sensorless Control Based on the Uncertainty and Disturbance Estimator for IPMSMs With Periodic Loads

The research object in this paper is a single rotor compressor based on the interior permanent magnet synchronous motors (IPMSM) with sensorless control, whose load fluctuates periodically with the mechanical angle. The periodically fluctuating loads can cause obvious periodic fluctuations of motor speed. Meanwhile, the traditional phase-locked loop (PLL) can lead to obvious phase shift and amplitude attenuation. Therefore, the controller is designed from two aspects to make the motor run stably at low speed under the periodic load. Firstly, a speed control based on the uncertainty and disturbance estimator (UDE) is proposed to suppress speed fluctuations caused by the periodic load. Secondly, the phase-locked loop based on the resonance control (R-PLL) is proposed to compensate for the periodic angle and speed errors caused by the phase shift coming from the PLL under the periodic load. The experimental results show that the sensorless control based on the R-PLL and UDE can effectively improve the estimated angle accuracy and suppress the speed fluctuations, making the motor run stably at low speed under periodic loads.

Discrete Robust Control of Robot Manipulators Using an Uncertainty and Disturbance Estimator

AbstractThis article presents the design of a robust observer based on the discrete-time (DT) formulation of uncertainty and disturbance estimator (UDE), a well-known robust control technique, for the purpose of controlling robot manipulators. The design results in a complete closed-loop, robust, and controller–observer structure. The observer incorporates the estimate of the overall uncertainty associated with the plant, in order to mimic its dynamics, and the control law is generated using an auxiliary error instead of state tracking error. A detailed qualitative and quantitative stability analysis is provided, and simulations are performed on the two-link robot manipulator system. Further, a comparative study with well-known control strategies for robot manipulators is presented. The results demonstrate the efficacy of the proposed technique, with better tracking performance and lower control energy compared to other strategies.

UDE-Based Current Controller With Enhanced Grid Frequency Fluctuation Adaptability for LCL-Type Grid-Tied Inverters

The uncertainty and disturbance estimator (UDE) with a time delay filter has been widely used for inverter systems due to its satisfactory harmonic rejection performance. However, the control performance may be affected when the grid frequency fluctuates. In this article, a modified UDE (FUDE) is proposed to improve the grid frequency fluctuation adaptability of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LCL$</tex-math></inline-formula> grid-tied inverter systems, which contains a compound filter consisting of time delay, zero-phase low-pass, and high-pass filters. Compared with the traditional time delay UDE, it can offer a wider notch bandwidth at the selected frequencies. A dual-loop controller is built in the separate-structure-type UDE-based controller, containing an outer loop PR controller and an inner loop FUDE. The sufficient conditions for the system stability are obtained by the small-gain theorem. The comparison experiments of the dual-loop controller with the proposed FUDE and the conventional time delay zero-phase low-pass UDE are performed on a 2-kW experimental platform, and the results demonstrate the effectiveness of the proposed FUDE.

Cascade-modified uncertainty and disturbance estimator–based control of quadrotors for accurate attitude tracking under exogenous disturbance

This paper presents a cascade-modified uncertainty and disturbance estimator (CMUDE)–based controller. The attitude-tracking performance of a quadrotor would be greatly degraded due to two factors: (1) a nonideal actuator and (2) an exogenous disturbance. Based on these factors, an actuator model is accurately identified using a thrust test platform. Then, a cascade-structured CMUDE controller is designed through the backstepping method, so the disturbances from different channels can be separately dealt with. The idea of the uncertainty and disturbance estimator (UDE) is then applied to dynamically compensate for not only the exogenous disturbance but also the time delay in the actuator dynamics. Benefitting from that, the problem of mismatched disturbances is solved by defining virtual control inputs; furthermore, the nonlinear term in the attitude kinematics is included in the model uncertainty to be canceled instead of being ignored. When tracking attitude commands, the exogenous disturbance and model uncertainty along with the time delay of the actuator are estimated and compensated for by the proposed controller, so the performance of the disturbance rejection is significantly improved while maintaining a quick response to the rapidly changing commands. To verify that, the proposed controller is implemented on a quadrotor platform to carry out exhaustive experiments and compared with the mainstream cascade PID (CPID) controller, the classic UDE-based controller, and the latest modified-UDE (MUDE)–based controller. The results show that our control strategy achieves a better disturbance rejection and more accurate tracking performance than other controllers.

Robust tuning of uncertainty and disturbance estimator-based control for stable processes with time delay

To expand the potential of uncertainty and disturbance estimator (UDE)-based control in practical application to most industrial stable processes, this paper proposes a convenient yet robust tuning rule according to the widely used first-order plus time delay (FOPTD) plant. The Smith predictor is first introduced to anticipate the delay-free output, which guarantees signal synchronizations in three control modules and enables remarkable restorations of nominal stability and performance. Then a second-order filter is employed in UDE to decouple the trade-off between disturbance rejection and noise attenuation. Based on this improvement and fixing both tracking speed and feedback gain to suggested patterns, the exhaustive evaluations for robustness against model distortion are executed through scanning the dimensionless filter bandwidth. The boundary demarcation triggered by the plunge of the continuous range of tolerable mismatched delays subsequently facilitates the formulation of an intuitive tuning rule with prescribed robustness. Its inherent model-based scaling property largely enables this rule to be implemented readily in industrial processes just like the proportional-integral-derivative (PID) controller. Several representative simulations are performed to demonstrate the merits of the proposed method over the related control strategies. And the promising prospect of the UDE-based control in the practical application is further illustrated by conducting a water level control experiment.

Robust Control Based on a Time-varying UDE for an Under-actuated 3-DOF Helicopter

The UDE (uncertainty and disturbance estimator) based control approach has received increasing attention in recent years due to its simplicity and effectiveness to attenuate the influence of uncertainties and disturbances. Despite this fact, the trade-off issue of its steady-state performance and transient performance has not been addressed explicitly. A novel time-varying UDE (TV-UDE) is proposed to resolve the trade-off issue in this paper. The main idea behind the design is to use a differential equation with a time-varying parameter, rather than a transfer function, to represent the underlying filtering relationship. By changing the UDE gain smoothly, the performance limitations of a fixed-gain UDE are avoided. Moreover, the TV-UDE is reduced to a classic UDE when its parameter value is finally frozen. Two design specifications are provided to guide the design of the time-varying parameter. We then apply this design to the angular trajectory tracking problem of a 3-DOF helicopter model, and reveal the four time-scales property of the closed-loop dynamics. The advantages of the proposed approach are shown by simulation comparisons.

Composite disturbance rejection and tracking control design for fuzzy model based systems with cyber attacks

This paper addresses the output tracking control problem and lumped disturbance rejections for nonlinear cyber-physical systems described by fuzzy model systems with parametric uncertainties, external disturbances and cyber attacks. Especially, to assure both the robust tracking and lumped disturbance estimation performances, an uncertainty and disturbance estimator (UDE)-based modified repetitive control (MRC) protocol is devised for the addressed control system. In detail, the impact of unpredictable factors, such as disturbances and uncertainties in the system model is robustly tackled by the UDE method with an appropriate filter. To be specific, the MRC protocol is amalgamated into the UDE technique for estimating the changes that occur in the afore-said unpredictable factors to ensure the asymptotic tracking performance of the system under study. Notably, to improve the tracking precision, the parallel distributed compensation strategy and the estimated lumped disturbance signal are incorporated into the control design. Meanwhile, the cyber attacks are depicted in this study as a stochastic variable that adheres to the Bernoulli distribution. Further, the required controller gain matrices are determined to affirm the robust asymptotic tracking performances with the aid of Lyapunov stability theory. Eventually, the method proposed in this paper is validated through the simulation results with two numerical examples, which include the continuous stirred tank reactor (CSTR) plant model. Further, the result reveals that the proposed control law provides the minimum tracking error when compared with an existing work.

UDE-based robust nonlinear control of the buck-boost power converter

ABSTRACT The buck-boost converter is a non-minimum phase system that presents major challenges in the design of stabilising controllers. A robust nonlinear controller based on the uncertainty and disturbance estimator (UDE) scheme is developed to tightly regulate the output voltage of the converter, subject to unknown input voltage, load resistance, parasitics and parametric uncertainties. A simple procedure is developed that systematically select the controller gains to achieve a satisfactory output response. Using simulation, the effectiveness of the proposed controller is validated and compared to a recent robust nonlinear proportional-integral (PI)-type controller.