Showing papers by "Kang-Zhi Liu published in 2018"

An Improved Equivalent-Input-Disturbance Approach for Repetitive Control System With State Delay and Disturbance

Abstract: An improved equivalent-input-disturbance (EID) approach is devised to enhance the disturbance-rejection performance for a strictly proper plant with a state delay in a modified repetitive-control system. A gain factor is introduced to construct an improved EID estimator. This increases the flexibility of system design and enables the adjustment of the dynamical performance of disturbance rejection. Moreover, the commutative condition, which is widely used for the conventional EID estimator, is avoided. Thus, it reduces the conservativeness of design by removing the constraints imposed by the commutative condition. The system is divided into two subsystems, and the separation theorem is applied to simplify the design. For one subsystem, the delay information on both the modified repetitive controller and the plant is used to reduce the conservativeness of stability condition. The resulting linear matrix inequality (LMI) is used to find the gain of the state-feedback controller. Another LMI is derived to design the gains of the state observer and the improved EID estimator for the other subsystem. A case study on a metal-cutting system validates the superiority of the developed method.

Robust Tracking and Disturbance Rejection for Linear Uncertain System With Unknown State Delay and Disturbance

Abstract: A robust tracking control method is presented in this paper for an uncertain plant with an unknown state delay and an exogenous disturbance. The effects of the uncertainties, the delay, and the exogenous disturbance are treated as a total disturbance; thus, the construction of the observer does not need the delay information. The system design is divided into the design of the gains of the state-feedback controller as well as the design of the gains of the observer and the improved equivalent-input-disturbance (EID) estimator. The pole-assignment method is used to design the gains of the state-feedback controller of a simplified system. A robust-stability condition in the form of a linear matrix inequality is derived to determine the gains of the observer and the improved EID estimator. Since the devised Lyapunov functional is of a more general form than those in existing EID-based methods and the restrictive commutative condition is avoided in this design, the developed design method is less conservative. Finally, comparisons of the developed method with a sliding-mode control method for a matched disturbance and a conventional EID-based method for an unmatched disturbance illustrate the validity and superiority of the developed method.

Analysis and Design of Filters for the Energy Storage System: Optimal Tradeoff Between Frequency Guarantee and Energy Capacity/Power Rating

Abstract: This paper proposes a new filter design method that can smooth the wind power generation output while keeping the energy capacity and power rating of the energy storage system (ESS) small. Wind power generation causes frequency fluctuations in power systems. Therefore, in many cases, the ESS is needed for smoothing the fluctuations of wind generator output. In order to improve the performance of the ESS controlled with the conventional low-pass filter (LPF), this paper investigates other types of filters. First, the relations between filter and energy capacity/power rating of the ESS are disclosed through the worst-case study. Then, the frequency fluctuation analysis of the power system with respect to the wind power generation is carried out in order to realize a pin-point smoothing. Finally, a metaheuristic optimization method is proposed for the multiobjective design of the filter based on these relations so as to achieve an optimal tradeoff between the output smoothing and energy capacity/power rating of the ESS. It is shown via simulation analysis with real wind data that the proposed method can suppress the frequency fluctuation of the power system effectively with the ESS having smaller energy capacity and power rating.

Robust Performance Design for Systems With Output Strictly Passive Uncertainty

Abstract: In the robust control of systems with dynamic uncertainty, there are mainly two methodologies: one based on the gain bound of uncertainty and the other based on the phase bound. The well-known small-gain approach belongs to the former and is able to conduct nonconservative robust performance design by using $\mu$ -synthesis. Meanwhile, the study on the latter case is still under way, and only a positive-real method was proposed by the authors recently to handle the robust performance synthesis. This paper aims at improving the positive-real method. The key idea is to model a passive uncertainty not only by its phase bound but also utilizing its largest gain. This is achieved by applying the notion of output strict passivity well known in nonlinear control. Furthermore, the negative-imaginary uncertainty can be treated in this framework more naturally. A case study on a vibration system validates the advantage of the present method.

To Improve Drilling Efficiency by Multi-objective Optimization of Operational Drilling Parameters in the Complex Geological Drilling Process

Abstract: Operational drilling parameters optimization is necessary for increase drilling efficiency in complex geological drilling process. There are four key objectives to evaluate the drilling efficiency, including drilling cost, rate of penetration (ROP), drill bit life, and drill bit's specific energy. In this paper, we proposed a multi-objective optimization method to improve drilling efficiency in the complex geological drilling process considering all the four key objectives. Firstly, the characteristics of complex geological drilling process and optimization problems are analyzed to find the vital process parameters and problems for drilling efficiency optimization. Then, a multi-objective optimization model which combines the four key objectives is established. Finally, an improved nondominated sorting genetic algorithm (improved NSGA-II) is used to optimize the operational drilling parameters include weight on bit (WOB) and rotational speed (RS) to make drilling efficiency improved. The real case results demonstrate that our method increases the drilling efficiency in four key objectives and saves the simulation time. The proposed method provides the foundation for intelligent optimization control in complex geological drilling process.

Enhancement of disturbance-rejection performance of uncertain input-delay systems: a disturbance predictor approach

Abstract: To suppress disturbances in uncertain tracking control systems with an input-delay, a disturbance predictor based on a high-order generalised extended-state observer is devised in this study. A smooth enough approximation of the disturbance is first made, then the approximation ahead of delay-time is predicted. The construction of the disturbance predictor is based on a truncated Taylor polynomial. To facilitate the analysis and design of the closed-loop control system, control gains for the nominal plant are designed in advance. Then the stability analysis for the closed-loop system is conducted, which yields a robust stability condition. As the disturbance predictor takes into consideration the influence of the input-delay, the presented method enables the enhancement of the disturbance-rejection performance. Finally, comparisons of the developed method with major methods in this field are conducted to validate the developed method and to demonstrate its advantages.

Robust Control of Contact Force for Pantograph-Catenary System

Abstract: To improve the quality of the current collection transmitted to a high-speed train, based on a pantograph model with a three-degree of freedom, two methods are presented to suppress the vibration of the pantograph-catenary system. One is $H_{\infty}$ robust control, and the other is improved equivalent-input-disturbance (IEID)-based control. For the $H_{\infty}$ robust control, a robust stability condition together with a performance criterion in the frequency domain is given to obtain a dynamic controller. For the IEID-based method, the LQR regulation combined with LMI technique and Lyapunov stability theory is used to design the control gains of the state-feedback controller, the state observer and the IEID estimator in the time domain. Simulations show that these two methods achieve good vibration suppression performance in the steady state.

Model Predictive Cell Zooming for Energy-Harvesting Small Cell Networks

Abstract: This paper addresses the real-time control of transmission power for small cell base stations (SBSs) exploiting energy- harvesting sources. We employ model predictive control and optimize an objective function that contains the number of users with a given quality of experience and the average state of charge. We first determine the number of active SBSs in the viewpoint of energy efficiency and then approximate the objective function. Finally, we illustrate the proposed method through a numerical example, comparing it with a static method based on statistical information.

Optimal Automatic Formation Control for Two-Wheeled Vehicles Using Model Predictive Control with Temporal Logic Constraints

Abstract: Recently,formation control in which multiple ve- hicles travel while maintaining a certain shape or switching some forms has been studied. There are many advantages compared with a single vehicle such as more efficient, scalauility, versatility, and redundancy against individual failure. However, many papers assume a sufficiently large space, and a few papers consider the surrounding environment. Hence, it is necessary to consider environment in limited space for safety. Therefore, this paper proposes an optimal automatic formation control method according to the environment. In our approach, the edge of course are defined as the environment and optimal formation is automatically determined among predefined formations in real time using model predictive control. In addition, we introduce temporal constraints so that we can specify formation when a certain formation is requested during cruising.

Optimal Self-triggered Control in Networked Control Systems

Abstract: Networked control systems (NCSs) have attracted attentions and have a great advantage that NCSs can reduce costs of installation and have extensive industrial application. On the other hand, the disadvantage that the control performance can be degraded and the plant can be destabilized is happening. Furthermore, data transmission means waste of communication resource and transmission energy when it is not necessary to transmit the packet data. In order to solve the problem, an event-triggered control and self-triggered control are studied. Although these method is attractive, most of recent studies have only focused on an analysis of stability. Therefore, we proposed self-triggered control based on model predictive control to improve the performance furthermore. In addition, Kalman filter is extended and event-driven estimator is derived and employed to the systems. This strategy can reduce transmission rate drastically while guaranteeing the control performance. Eventually, the results are shown and displayed by demonstrating simulations.

Robust Disturbance- Rejection for a Class of Nonlinear Systems

Abstract: This paper presents a robust disturbance-rejection method for a system with a time-variant nonlinearity and a disturbance. The nonlinear dynamics which satisfies Lipschitz condition is unknown. The disturbance is also unknown but bounded. Taking advantage of stable inversion, an improved EID (IEID) is utilized to deal with the total effect of the nonlinearity and disturbance. To reduce the conservativeness of system design, a feedback control gain is obtained in prior to the control gains of the observer and the IEID estimator. Then a Lyapunov functional considering the information of the nonlinearity is constructed and based on that, a linear-matrix-inequality (LMI)-based stability condition is derived to acquire the gains of the observer and the IEID estimator. Two tuning parameters are introduced in the LMI to enable the adjustment of disturbance-rejection performance. Finally, simulations for a flight control system show that the developed method has good robustness. Comparisons with disturbance-observer-based control (DOBC), composite DOBC and $H$ oo control, and conventional EID-based control methods demonstrate the effectiveness of the developed method and its advantages over others.