Abstract

This paper investigates the robust finite-time control problem for flexible spacecraft attitude tracking maneuver in the presence of model uncertainties and external disturbances. Two robust attitude tracking controllers based on finite-time second-order sliding mode control algorithms are presented to solve this problem. For the first controller, a novel second-order sliding mode control scheme is developed to achieve high-precision tracking performance. For the second control law, an adaptive-gain second-order sliding mode control algorithm combing an adaptive law with second-order sliding mode control strategy is designed to relax the requirement of prior knowledge of the bound of the system uncertainties. The rigorous proofs show that the proposed controllers provide finite-time convergence of the attitude and angular velocity tracking errors. Numerical simulations on attitude tracking control are presented to demonstrate the performance of the developed controllers.

1. Introduction

In recent years, considerable attention has been focused on spacecraft attitude control problems. Attitude control systems are required to offer the present generation of spacecraft with attitude maneuver, tracking, and pointing capabilities. Since the attitude dynamics of spacecraft is coupled and highly nonlinear, the attitude controller designs are usually difficult. Besides, the effect of motion of the elastic appendages makes the control problem more complicated. In the absence of external disturbances, the modal-independent proportional-derivative (PD) controller proposed in [1] can achieve asymptotical stability of both the attitude and angular velocity. In practical situations, the model parameters of the spacecraft may not be exactly known and the spacecraft is always subject to external disturbances. Thus, the attitude control problem with uncertainties and external disturbance has also attracted a great deal of attention. Various nonlinear robust control approaches [2–4] have been proposed for solving the attitude tracking control problem of flexible spacecraft. These control schemes include adaptive control [5, 6], sliding mode control [7, 8], output feedback control [9, 10], optimal control [11, 12], and intelligent control [13].

Among these methods, sliding mode control (SMC) has been shown to be a potential approach when applied to a system with disturbances which satisfy the matched uncertainty condition [14]. Robust attitude controllers of flexible spacecraft based on the SMC scheme have been proposed in [15, 16]. These control laws can achieve global asymptotic stability and provide good tracking results. However, these controllers were designed based on an asymptotic stability analysis which implies that the system trajectories converge to the equilibrium with infinite settling time. It is well known that finite-time stabilization of dynamical systems may provide a faster disturbance attenuation besides giving faster convergence to the required orientation. A recently developed technique for finite-time stabilization is the terminal sliding mode (TSM) method [17, 18] which can be used to design a controller that will guarantee a finite-time convergence to the origin. In [19, 20], the attitude motion of flexible spacecraft has been studied and the TSM method was used to design finite- time controllers. However, the TSM method usually provides lower tracking precision when compared with second-order sliding mode control (SOSMC) schemes [21, 22]. This technique preserves the robustness ability of SMC and also yields improved accuracy and performance. Various real-life applications have been controlled in a practical implementation of SOSMC schemes (e.g., see [23–25]). The SOSMC strategies have been successfully applied to attitude tracking controller designs for a rigid spacecraft in [26, 27], but these schemes have been rarely used for attitude tracking control of flexible spacecraft.

In this paper, by virtue of SOSMC designs, the proposed attitude tracking control laws can guarantee the convergence of attitude tracking errors in finite-time. First, a novel finite-time SOSMC scheme is designed to achieve fast and accurate tracking responses. Next, to obtain the second control law, we combine the adaptive law with the SOSMC scheme proposed in [27]. This controller relaxes the requirement of prior knowledge on the bound of uncertainties. With the time scaling approach [28], the control parameters can be tuned by using only one variable.

The main contributions of this paper are as follows.(I)Robust finite-time control algorithms based on SOSMC schemes have been rarely studied for attitude tracking maneuver of a flexible spacecraft. The finite-time stability of the proposed control laws is analyzed using Lyapunov stability concepts.(II)A novel adaptive law for the gains of the SOSMC algorithm has been rarely developed by using the time scaling approach. The presented control method does not need a priori knowledge of the uncertainty and disturbance bounds.

This paper is organized as follows. In Section 2, the dynamic and kinematic equations governing the attitude model [29, 30] are described and the control design problem is formulated. Section 3 presents a novel SOSMC control algorithm for a flexible spacecraft. The sliding manifold is chosen and the sliding control law is studied and a proof of finite-time convergence of this controller is given. Section 4 proposes an adaptive-gain SOSMC law. The stability of the closed-loop system is analyzed. A numerical example of spacecraft tracking maneuvers is presented in Section 5 to verify the usefulness of the proposed controllers. In Section 6, we present conclusions.

2. Nonlinear Mode and Problem Formulation

2.1. Spacecraft Attitude Dynamics and Kinematics

The unit quaternion is adopted to describe the attitude of the spacecraft for global representation without singularities [29]. The unit quaternion is defined by where is a unit vector called the Euler axis, denotes the magnitude of Euler axis rotation and and are the vector components and the scalar of the unit quaternion, respectively. They are subject to the constraint . Consider the first time derivative of . The kinematic equations are given by [29, 30] where is a identity matrix, and is a skew-symmetric matrix:

2.2. Relative Attitude Error Kinematics

We explain briefly the attitude error using quaternions. We define here the desired quaternion with  . Also the attitude error with . Using the quaternion multiplication law, we obtain subject to the constraint

The kinematic equation for the attitude error is expressed as [29, 30]

2.3. Flexible Spacecraft Dynamics

The equation governing a flexible spacecraft is expressed as [3] where is the total inertia matrix of the spacecraft, is the modal displacement, and is the coupling matrix between the central rigid body and the flexible attachments. represents the angular velocity vector and is a skew-symmetric matrix with a formula similar to . is the control input, represents the external disturbance torque, and and denote the stiffness and damping matrices, respectively, which are defined as with damping and natural frequency .

If terms and are considered as lumped perturbations to rigid body dynamics, (7) can be obtained as where may be considered as the lumped perturbation. Let the angular velocity error be defined as ; then the error dynamics are given by where .

Using (6), one has with .

Define and   and consider the time derivative of (11). One can obtain [3] where

Throughout the remainder of this paper, the following are assumed.

Assumption 1. The elastic oscillation and its rate are supposed to be bounded; that is to say, and are bounded during the whole attitude tracking process.

Let and ; the spacecraft model (12) can be written as where , , and .

Assumption 2. The first time derivative of the disturbance vector to the spacecraft system in (12) is assumed to be bounded and it satisfies the following condition: where is a positive constant.

2.4. Problem Statement

In this work, we consider tracking maneuvers. The control objective is to realize desired rotations of flexible spacecraft in the presence of external disturbances. In other words, we shall find a controller subject to (14) such that, for all initial conditions, the desired rotations are achieved as follows: where is a positive constant. Note that, when , we have , due to the constraint relation (5).

2.5. Finite-Time Stability

We now restate the concepts related to finite-time stability [31, 32].

Definition 3 (see [31]). Consider a time invariant system in the form of where is continuous on an open neighborhood of the origin. The equilibrium of the system (17) is (locally) finite-time stable if (i) it is Lyapunov stable, in , an open neighborhood of the origin, with , (ii) it is finite-time convergent in , that is, for any condition , there is a settling time , such that every solution of the system (17) is defined with for and it satisfies and if . Moreover, if , the origin is globally finite-time stable.

Definition 4. Consider a controlled system with . It is finite-time stabilizable if there is a feedback law such that is a (locally) finite-time stable equilibrium of closed-loop system.

Lemma 5 (see [32]). Suppose that the origin is a finite-time stable equilibrium of (17) and the settling time function is continuous at zero, where is continuous. Let be defined as in Definition 3 and let . There exists a continuous scalar function such that (i)   is positive definite and (ii)   is real valued and continuous on and there exists such that

Lemma 6 (Feng et al. [18]). For any numbers , , and , an extended Lyapunov contion of finite-time stability can be given in the form of fast terminal sliding mode as where the settling time can be estimated by

3. SOSMC Algorithm

In this section, a novel SOSMC algorithm is proposed to accomplish attitude tracking control of a flexible spacecraft. Under Assumptions 1 and 2, this control scheme is designed such that the attitude tracking error and the angular velocity error approach zero in finite time although uncertainties and external disturbances are taken into account. This new SOSMC law is achieved by a Lyapunov function approach, so the finite-time stability of the closed-loop system of the spacecraft model (14) is ensured. The goal is to enforce the sliding mode on the manifold where and is a positive constant. From (23), the first time derivative of is obtained as Then, it is followed by choosing a SOSMC law to force the system trajectory onto the sliding surface. Let and ; the proposed control law is defined as where . The gain matrices , , , and , with , , and , are positive gains. The function is defined as with and .

Substituting (25) into (24), the closed-loop dynamics is obtained as Let ,   and . The system (27) can be written in scalar form as Next, the proof of finite-time convergence to the origin is given.

Theorem 7. Under Assumptions 1 and 2 and the action of the control law (25), the state trajectories and converge in finite time to the region where and , are the results from the chosen gains.

Proof. We choose the Lyapunov function for analyzing the closed-loop system dynamics of (28). The time derivative of is given by which can be further written as where , , , and , .
Letting and , one obtains Letting and , we can change (32) into the following forms: From (33), if then the finite-time stability is still ensured, and hence, by Lemma 5, the state trajectories and of the system (28) converge to the region in finite time. From (33), if , then the finite-time stability is still ensured, and hence, by Lemma 5, the state trajectories and of the system (28) converge to the region in finite time. Therefore, the state trajectories and of the system (28) converge in finite-time to the region where and .

Remark 8. The accuracy of tracking errors and is determined by the control parameters , , , and , . To obtain the desired accuracy, it is necessary to reduce the size of the region (37) by increasing the values of the control parameters , , , and . However, using large values of these parameters leads to high magnitudes of control torques.

4. Adaptive-Gain SOSMC Algorithm

Next, the proposed adaptive-gain SOSMC scheme is designed by modifying the smooth SOSMC law presented in [27]. In stead of using the constant controller gains as presented in [27], the adaptive gains are considered in this paper. A novel adaptive law to tune these gains is presented. The main advantages of the proposed method are that only one parameter has to be tuned and this method relaxes the requirement of prior knowledge of the bound of system uncertainties.

The proposed control law is given by where and the adaptive gains , , , and are the design parameters defined as with , , , being positive constants. The adaptive law of the time-varying function is given by where and the initial value are positive constants.

Substituting (38) into (14), we obtain Next, the same procedure as presented in Section 3 is recalled. We can have the scalar form of (41) as

Theorem 9. Suppose that , , , are selected such that and Assumptions 1 and 2 hold. Then, all trajectories of the system (42) converge in finite time to the region where and the positive scalars and will be defined later.

Proof. Let the Lyapunov function be chosen as The selected Lyapunov function can be expressed as which can be written as where It satisfies where . The time derivative of is obtained as Substituting and into (50), we obtain
After lengthy algebraic manipulation, one obtains which can be written as where , , By Assumption 2 and using , one obtains
Using , we have
Using (49), the expression (56) becomes
From simplicity, we define where , , , and are positive constants. Thus, (57) can be written as
Because such that the term is positive in finite time, it follows from (59) that, if , the error system (42) will converge in finite time to the region In fact, we can choose , , , and such that . With , being sufficiently large, so is sufficiently small in finite time.