Robust Servomechanism Controller Research Articles

Observer-based robust servomechanism control of a three-phase induction motor drive

Induction motors (IMs) are widely used in industrial applications, but harmonic distortion in IM drives can lead to inefficiencies and reduced lifespan. Traditional control strategies and filtering methods have limitations in mitigating lower-order harmonics, particularly the 5th harmonic, which can generate reverse torque and increase overheating risk. Existing solutions often struggle to fully address these issues, especially in high-power applications with dynamic loading conditions. This paper proposes a control strategy to reduce voltage and current harmonics in a Z-source inverter (ZSI)-fed IM drive. The system incorporates an observer-based output feedback robust servomechanism controller, designed using Linear Quadratic Regulator (LQR) and Linear Matrix Inequality (LMI) techniques. It ensures closed-loop stability, precise tracking regulation, and effective disturbance rejection, while also featuring a speed adaptation mechanism for sensorless operation. Simulation results demonstrate significant harmonic mitigation, with the controller achieving 1.01% lower voltage THD and 1.03% lower current THD compared to conventional approaches. This improvement aligns with industry standards and outperforms several existing techniques. The proposed method contributes a comprehensive solution for harmonic mitigation in high-power IM drives, applicable to various industrial applications requiring high-quality power and precise speed control.

A generalized descriptor-system robust H∞ control of autonomous microgrids to improve small and large signal stability considering communication delays and load nonlinearities

This paper presents a descriptor system H∞ approach to enhance performance of robust H∞ controller and previously-reported decentralized robust servo-mechanism (DRSM) control scheme for autonomous voltage sourced converter (VSC)-based microgrids including multiple distributed energy resources (DERs). The power management system specifies voltage set points for local controllers and the frequency of each DER unit is specified by a hierarchical droop-based control structure. A descriptor system H∞ robust controller is designed based on a closed-loop representation of microgrid either with H∞ or DRSM controller for set point tracking, disturbance rejection and improving performance of microgrid for small/large-signal disturbances and nonlinear loads. Here, unlike some of the previous researches, the load current is modeled as disturbance and also communication time-delay is considered. The theoretical concepts of proposed control strategy, including mathematical modeling of microgrid, basic theorems, and design procedure are outlines. Then, design problem is formulated by a set of linear/bilinear matrix inequalities and then solved using a new iterative algorithm in the form of convex optimization problem. To demonstrate effectiveness of the proposed control scheme, offline time-domain simulation studies are performed on a multi-DER microgrid in MATLAB/Simulink environment and also the results are experimentally verified by OPAL-RT real time digital simulator.

Multivariable Servomechanism Controller for Autonomous Operation of a Distributed Generation Unit: Design and Performance Evaluation

A linear time-invariant (LTI) robust servomechanism controller for islanded (autonomous) operation of a distributed generation (DG) unit and its local load is proposed. The DG unit utilizes a voltage-sourced converter (VSC) as the interface medium. The controller design is obtained by introducing a new optimal controller design procedure, in conjunction with a proposed non-conservative robustness constraint. The proposed controller utilizes 1) an internal oscillator for frequency control and 2) a robust servomechanism controller (RSC) to regulate the island voltage. Despite uncertainty of the load parameters, the proposed controller guarantees robust stability and pre-specified performance criteria, e.g., fast transient response and zero steady-state error. The theoretical aspects of the proposed robust servomechanism controller including the existence conditions, design of the controller, and robust stability analysis of the closed-loop system are studied. Moreover, the performance of the robust servomechanism controller based on 1) simulation studies in the MATLAB software environment, and 2) experiments in a laboratory-scale setup, is presented in this paper. In particular, reference signal tracking and robustness of the closed-loop system with respect to the load parameter uncertainty are investigated. Both computer simulation studies and experimental results confirm that the proposed robust controller provides the specified performance characteristics of the closed-loop system.

Robust measures for a large platoon of vehicles using non-identical controllers

This paper studies the robustness of a platoon of identical vehicles subject to decentralized control, where each control agent is assumed to only have knowledge of the distance between itself and its immediate forward neighbour. Two different types of controllers: (i) a decentralized PID controller and (ii) a decentralized robust servomechanism controller, are considered in the control of such a platoon of identical vehicles, where it is assumed that the models of the vehicles are subject to perturbation. In this case two types of perturbations are considered: (i) the perturbations applied to the vehicles are identical from vehicle to vehicle, (ii) the perturbations applied to the vehicles are non-identical. A study of two types of robustness measures is then considered: (i) the “real stability radius” robustness with respect to the perturbed closed loop system and (ii): the “string stability margin robustness” with respect to the perturbed closed loop system. Various numerical examples are presented to illustrate the type of results which may be obtained. In this case it is shown that the “string stability margin robustness” measures obtained are much more fragile than the corresponding “real stability robustness measures obtained.

Two Reconfigurable Flight-Control Design Methods: Robust Servomechanism and Control Allocation

Two methods for control system reconfiguration have been investigated. The first method is a robust servomechanism control approach (optimal tracking problem) that is a generalization of the classical proportional-plus-integral control to multiple input-multiple output systems. The second method is a control-allocation approach based on a quadratic programming formulation. A globally convergent fixed-point iteration algorithm has been developed to make onboard implementation of this method feasible. These methods have been applied to reconfigurable entry flight control design for the X-33 vehicle. Examples presented demonstrate simultaneous tracking of angle-of-attack and roll angle commands during failures of the fight body flap actuator. Although simulations demonstrate success of the first method in most cases, the control-allocation method appears to provide uniformly better performance in all cases.

Internal-model-based design of repetitive and iterative learning controllers for linear multivariable systems

Repetitive and iterative learning control are two modern control strategies for tracking systems in which the signals are periodic in nature. This paper discusses repetitive and iterative learning control from an internal model principle point of view. This allows the formulation of existence conditions for multivariable implementations of repetitive and learning control. It is shown that repetitive control can be realized by an implementation of a robust servomechanism controller that uses the appropriate internal model for periodic distrubances. The design of such controllers is discussed. Next it is shown that iterative learning control can be implemented in the format of a disturbance observer/compensator. It is shown that the resulting control structure is dual to the repetitive controller, and that both constitute an implementation of the internal model principle. Consequently, the analysis and design of repetitive and iterative learning control can be generalized to the powerful analysis and design procedure of the internal model framework, allowing to trade-off the convergence speed for periodic-disturbance cancellation versus other control objectives, such as stochastic disturbance suppression.

Multirate robust servomechanism controllers of linear delay systems using a hybrid structure

A multirate robust servomechanism controller design for a linear time delay system is presented. The proposed controller is constructed by using a preparatory continuous internal model followed by a multirate error feedback compensator. An optimal algorithm for choosing the controller parameters is also derived. The algorithm is derived by minimizing a squared deviation error performance index, and the practical optimal solution can simply be solved from a discrete-time algebraic Riccati equation. A distinctive feature of the algorithm is that an optimal pulse compensation is also employed to improve the transient characteristics. Such a pulse compensation tends to eliminate the undesired influence arising from the initial deviation from the ultimate steady-state. In particular, if the initial deviation from the ultimate steady-state is known, then the optimal pulse compensation can drive the system into the steady-state in a time no larger than the input delay time plus the sampling period.

A ripple free sampled-data robust servomechanism controller using exponential hold

In a general command tracking and disturbance rejection problem, it is known that a sampled-data controller using zero-order hold may only guarantee the asymptotic tracking at the sampling instances, but cannot smooth out the ripples between the sampling instances. In this paper, a sampled-data robust servomechanism controller using exponential hold is developed for guaranteeing the asymptotic tracking not only at, but also between, the sampling instances. In this development, a so called "internally-reducible" condition to characterize a class of robust servomechanism controllers is derived first, then the proposed controller is shown to be contained in this class. Generally speaking, a sampled-data structure using exponential hold can provide more design freedoms so that it tends to simplify the construction of a robust servomechanism controller and facilitate the implementation on digital computers. An example for a DC motor control is presented to illustrate the advantages of this approach.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An optimal algorithm for sampled-data robust servomechanism controller using exponential hold

A new structure of sampled-data robust servomechanism controller using exponential hold is developed. An optimal algorithm is also proposed for choosing the controller parameters of two important special classes. The algorithm is derived by minimizing a square-error performance index, and the solution can be solved from a discrete-time algebraic Riccati equation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Robust Servo-Electronic Controller for Brake Force Distribution

In this paper a control system which achieves ideal brake force distribution between the front and the rear wheels of a vehicle during normal braking is studied. Ideal brake proportioning is achieved by dynamically controlling the rear-wheel speed to track the front-wheel speed. Electro-hydraulic brake actuators, which are installed at the rear brakes of the vehicle, are used to modulate the brake-line pressure. A simple linear model of the actuators was developed. This model is derived from experimental response data. Based on this model, a robust servomechanism controller, which achieves ideal brake proportioning by rear-wheel speed control, is designed, implemented, and tested. Test results indicate that the robust servo-mechanism controller achieves a very good wheel speed tracking performance.

Application of the robust servomechanism controller to systems with periodic tracking/disturbance signals

The following problem is considered: given a system which has tracking/disturbance signals arising from a class of periodic signals with period T, say, assume that such a periodic signal has p dominant harmonic components contained in {ω1, l = 0, 1,…, v}. Then it is desired to find a finite-dimensional linear controller which gives exact robust asymptotic regulation for the case of sinusoidal signals with frequencies ω1, l = 0, 1,…, v, subject to certain controller gain magnitude constraints and/or gain margin tolerance constraints which may be imposed on the problem. This is called ‘the approximate robust servomechanism problem for periodic signals of the class {ω1, l = 0, 1,…, v}’. It is to be noted that such a problem cannot in general be solved by using high gain control synthesis methods. Existence conditions and a controller design method are given to solve the problem. A number of examples is given to illustrate the type of results that may be achieved.

Perfect control of the robust servomechanism problem

The problem of finding a robust servomechanism controller which achieves exact asymptotic tracking/regulation for a modeled class of reference and disturbance signals, arbitrarily good approximate error regulation (AGAER) for reference/disturbance signals which lie outside of this design class, and arbitrarily good transient response is considered. It is shown that a necessary condition that must be satisfied to accomplish this requirement is that the initial conditions of the servocompensator should be zero. An explicit algorithm for designing such controllers is then given. A simple controller, called the high gain servomechanism controller is also presented, which has the property that it gives AGAER and arbitrarily good transient response, when the robust exact asymptotic tracking/regulation requirement is relaxed. This controller shows that dynamics in a controller is not essential for achieving good error regulation in the presence of unknown unmeasurable disturbances. It is also shown that using the controllers proposed, exact error regulation occurs, for a larger class of design signals, called the extended disturbance/reference input signals, even if the conditions for perfect control are not satisfied. Some examples are included to illustrate the type of results obtained.

Robust two-time-level control strategy for sampled-data servomechanism problem

A two-level control strategy consisting of a fast first-level analogue controller for ensuring rapid reduction of tracking error under process disturbance without unacceptable overshoot and a slow second-level digital-analogue robust servo-mechanism controller to ensure asymptotic tracking with disturbance rejection is proposed. Thanks to the inclusion of samplers with an adequately chosen sampling period, the second-level controller comes into action essentially after the first-level controller has ceased its action. An interactive design procedure for the determination of the parameters of the controller is given. Simulation results comparing the proposed controller with conventional one-level controllers, such as the analogue, the digital and the digital-analogue controller, are presented.

New structure for an adaptive servomechanism controller

A new stable adaptive controller structure for ensuring asymptotic tracking with disturbance rejection is proposed. The controller structure is identical to the robust servomechanism controller structure when the adaptation is over. The process is assumed to be a linear minimal-phase process. The exogenous signals may be unbounded, but satisfy a known linear difference equation. The parameter adaptation algorithm ensures global convergence of the tracking error.

Reliability of the Robust Servomechanism Controller for Decentralized Systems

A study of the reliability of the robust servomechanism controller for decentralized systems is made. In particular, it is desired to find conditions for a υ-control agent system, so that there exists a solution to the robust decentralized servomechanism problem for the system, and such that when a failure occurs in controller j, the resultant failed system still has the property that regulation and tracking occurs for control agents 1, 2, ..., j-1, ∀jϵ[1, 2, ..., ν]. This is called the decentralized robust servomechanism problem with sequential reliability. Necessary and sufficient conditions to solve the sequential reliability problem are given and an algorithm to construct sequential reliable robust controllers is given. Some numerical examples are included to illustrate the results.