Mathematical Problems in Engineering

Volume 2017, Article ID 7368092, 7 pages

https://doi.org/10.1155/2017/7368092

## Adaptive Control of a Class of Switched Nonlinear System with Partial State Constraints Using a Barrier Lyapunov Function

College of Information Science and Engineering, Northeastern University, Shenyang, China

Correspondence should be addressed to Enchang Cui; moc.liamtoh@iuc_mrahcdna

Received 27 March 2017; Revised 29 June 2017; Accepted 16 July 2017; Published 28 August 2017

Academic Editor: Asier Ibeas

Copyright © 2017 Enchang Cui et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper discusses partial state constraint adaptive tracking control problem of switched nonlinear systems with uncertain parameters. In order to ensure boundedness of the outputs and prevent the states from violating the constraints, a barrier Lyapunov function (BLF) is employed. Based on backstepping method, an adaptive controller for the switched system is designed. Furthermore, the state-constrained asymptotic tracking under arbitrary switching is performed. The closed-loop signals keep bounded when the initial states and control parameters are given. Finally, examples and simulation results are reported to illustrate the effectiveness of the proposed controller.

#### 1. Introduction

There is a strong industrial background of switched system in various fields. And for exactly that reason, many researchers have discussed the theoretical and applied research of switched system, and some of them have achieved commendable results in the last decade [1–3]. Since nonlinearity is the nature of the universe, more and more attention of switched nonlinear system has been drawn by control field [4–6]. It is also worth mentioning that a class of switched nonlinear system in lower triangular form is considered in [7], of which the backstepping method is used and a common virtual control is constructed to achieve the aim. Nevertheless, relatively accurate parameters are required by most control strategy for switched nonlinear system. Apparently, these desirable demands could not be met in practical terms.

Constraints are important issues in the study of physical systems, the authors in [8, 9] emphasized this problem in nonlinear saturation and performance and safety specifications, respectively. Taking a practical example for illustrative purposes, the attitude-control mass expulsion system of electrostatic microactuators is confronted with constraints, owing to the necessity of both the position and the speed of the movable electrode to be controlled. However, the development of constrained control is restrained by meeting the practical requirement. In this scenario, the extensive attention of the design and the analysis of constrained control has been increased. Recently, barrier Lyapunov functions (BLFs) have been proposed to solve the control problem of nonlinear systems. The state of the system will be not contradictory to the constraints [10–13] utilizing BLFs. Although the BLF is proven to settle constrained control problems [14], the parameters of switched nonlinear systems are expected to be deterministic.

Moreover, parameter uncertainties are widespread in realistic systems; hereon it has already been reported that adaptive control is an effective method to deal with such uncertain. On the one hand, remarkable achievements have been obtained from the research on adaptive control of nonlinear system. In [15–17], for instance, the control method is proposed and the preferable control performances are provided aiming at nonlinear time-delay systems. On the other hand, many researchers study the adaptive control problem of switched systems with uncertain parameters [18–20] with the development of adaptive control, and preliminary results have been obtained [21, 22]. In [23], an adaptive tracking controller is designed for switched stochastic nonlinear systems with unknown actuator dead-zone; the satisfying control performance is obtained as well. Among the aforementioned works, few authors have addressed the important issue that considering the uncertainty and the constraint together. Hence, the approach to design constraint adaptive controllers for switched nonlinear systems has been not reported to the best of our knowledge.

In the present paper, the adaptive tracking control problem of a class of switched nonlinear system with partial state constraints is solved. The progressive state tracking would not violate constraint conditions and all signals would be bounded when the parameter increases to infinitely great and approaches a certain value based on BLF. The control effectiveness of BLFs is verified by the comparative simulation results with quadratic Lyapunov functions (QLFs).

The remainder of this paper is organized as follows. Section 2 formulates the control problem. The partial state constraint problems of barrier Lyapunov functions are elucidated. Section 3 develops the adaptive controllers with state constraints; the stability analysis and the proof process are presented as well. Section 4 corroborates the expected effectiveness of the proposed controller by means of selected simulation results. Finally, Section 5 concludes this paper.

#### 2. Problem Statements and Preliminaries

##### 2.1. Problem Formulation

Consider a class of the switched nonlinear systems with uncertain parameters in the following form:where are the unknown piecewise constant parameters. is a non-Zeno switching signal which is right continuous. is the control input. and are smooth vector fields with and .

In addition, is unknown switched nonlinear function which can be linearly parameterized aswhere is smooth function and is a vector of uncertain parameters satisfying with known compact set . The parameters and , the switching time instants , , and the switching index are all unknown. There exist positive constants such that for all due to smoothness property.

According to (1) and (2), system (1) can be transformed in to the following form:The control target is to design an adaptive controller for system (1) such that tracks a desired trajectory asymptotically; that is, and , is a positive constant. Moreover, in order to make the problem here more tractable, we give the following assumption, which is common but practical.

*Assumption 1 (see [24]). * for all ’s have the same sign and whose common lower bound is known, that is, .

##### 2.2. Preliminaries

*Definition 2 (see [12]). *A barrier Lyapunov function (BLF) is a scalar function , defined with respect to the system on an open region containing the origin, that is, continuous, positive definite, has continuous first-order partial derivatives at every point of , has the property as approaches the boundary of , and satisfies along the solution of for and some positive constant .

According to previous description, we can choose a BLF candidate as follows:where denotes the natural logarithm. is the constraint on and satisfies the condition . Thus it can be seen that is positive definite, which grows to infinity when its argument approaches to its finite limit .

Lemma 3 (see [24]). *For function , a smooth function and an unknown positive constant can be found:*

Lemma 4 (see [10]). *For any positive constant , any satisfying , we have*

In order to make the problem more analysable and tractable, practical assumption is given for the adaptive state controller.

*Assumption 5 (see [24]). *We can find a smooth function such that

#### 3. Main Results

In this section, an adaptive controller is designed based on backstepping method by utilizing a BLF for systems (3).

*Step 1. *Define for as the tracking error which is . Consider the first component of the system (3) and we have the first-order partial derivative asChoose the following BLF candidate:where , denotes the estimate of , and . We design a stabilizing function and an adaptive law aswhere is a positive gain. Besides, let . On the basis of (10) and Assumption 1, we know that the time derivate of satisfieswhere can be found that .

*Step 2. *Consider the second component of system (3). Since , we can find a satisfying and choose the following BLF candidate:The stabilizing function and the adaptive law at this step are designed asUsing (13) and Assumption 1, we know that the time derivate of satisfies For the general case, we employ the BLF when :By the same token, we design stabilizing functions and adaptive laws:Finally, we obtain the stabilizing function and the adaptive law of step :

Theorem 6. *Considering system (3), the adaptive state feedback controller (18) and the adaptive law (19) can guarantee and under arbitrary switching signal and is a positive constant.*

*Proof. *Choose the BLF as follows:According to the adaptive state feedback controller (18) and the adaptive law (19), we obtain the time derivative of the BLF:It can be seen that all the signals are bounded. By Barbalat’s Lemma, will converge to zero. According to the nature of BLF, we can guarantee and , among them being a positive constant. The output of the system tracks the desired trajectory asymptotically and certain states are constrained.

The design procedure of the proposed control scheme could be viewed from the block diagram in Figure 1.