Mathematical Problems in Engineering

Volume 2016 (2016), Article ID 5641478, 11 pages

http://dx.doi.org/10.1155/2016/5641478

## Robust Control of Underactuated Systems: Higher Order Integral Sliding Mode Approach

^{1}Department of Electrical Engineering, Capital University of Science and Technology (CUST), Kahuta Road, Express Highway, Islamabad 44000, Pakistan^{2}Department of Electrical Engineering, The University of Lahore (UOL), Japan Road, Express Highway, Islamabad 44000, Pakistan^{3}Department of Mechatronics Engineering, International Islamic University, 50728 Kuala Lumpur, Malaysia^{4}Center for Advanced Studies in Telecommunications, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan

Received 25 September 2015; Revised 8 January 2016; Accepted 12 January 2016

Academic Editor: Wenguang Yu

Copyright © 2016 Sami ud Din 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 presents a robust control design for the class of underactuated uncertain nonlinear systems. Either the nonlinear model of the underactuated systems is transformed into an input output form and then an integral manifold is devised for the control design purpose or an integral manifold is defined directly for the concerned class. Having defined the integral manifolds discontinuous control laws are designed which are capable of maintaining sliding mode from the very beginning. The closed loop stability of these systems is presented in an impressive way. The effectiveness and demand of the designed control laws are verified via the simulation and experimental results of ball and beam system.

#### 1. Introduction

The control design of underactuated systems was the main focus of the researchers in the current and last decade. These systems, by definition, contain less number of control inputs/actuators as compared to the degree of freedom [1]. This feature makes them quite different from the other nonlinear plants where the systems operate with the same number of inputs and outputs, the so-called fully actuated systems. The control design of these systems is quite demanding because of their vital theoretical and practical applications in the areas of aerospace systems, marine systems, humanoids, locomotive systems, manipulators of different kinds, and so forth [2]. This family also includes ball and beam system [3], TORA (translational oscillator with rotational actuator) [4], and inverted pendulum system [5]. These systems are used in order to have minimum weight, cost, and energy usage while still retaining the key features of the processes. In addition, another significant feature of underactuated systems is less damage in case of collision with other objects which in turn provides more safety to actuators [6]. Underactuation can be raised due to the hardware failure; this hardware solution to actuator failures can be achieved by equipping the vehicle with redundant actuators [2]. Note that, in case of fully actuated systems, there exists a broad range of design techniques in order to improve performance and robustness. These include adaptive control, optimal control, feedback linearization, and passivity. However, it may be difficult to apply such techniques in large class of underactuated systems because sometimes these systems are not linearizable using smooth feedback [7] also due to the existence of unstable hidden modes in some systems. Brockett [8] also provided a necessary condition for the hold of stable smooth feedback law, but this condition is not satisfied in the majority of underactuated systems. Nevertheless, control design experts have employed approximate feedback linearization [9–11] and backstepping control [12]. Passivity-based methodology is also used to control such systems but the main drawback in this technique is its narrow range of applications [13]. Sliding mode control is also proposed for the class of underactuated systems [6] but the problem with sliding mode control is presence of chattering.

The aforementioned design strategies were quite suitable and resulted in satisfactory results but it is worthy to note that the system often becomes too sensitive to disturbance in the reaching phase of sliding mode strategy that the system may even become unstable. Therefore, in order to get rid of this issue the integral sliding mode strategy was proposed [14–16]. In this paper a robust integral sliding mode control (RISMC) approach for underactuated systems is proposed. The benefit of this strategy is enhancement of robustness from initial time instant. It also suppresses the well-known chattering phenomenon across the manifold. Before the design presentation, the system is suitably transformed into special formats. An integral sliding mode strategy is proposed for both the cases along with their comprehensive stability analysis. The proposed technique is practically implemented on the ball and beam system to authenticate the affectivity and efficiency of the designed algorithm. Note that in this paper our contributions are twofold. The first one is the development of the RISMC and the second one is the practical results of the system on the said system. The rest of the paper is organized as follows. In Section 2, the problem is formulated into two special formats which further simplify the design methodology. In Section 3, the integral sliding mode strategy for both the cases is discussed in detail accompanied by their respective stability analysis in terms of Lyapunov theory. Section 4 presents the development of the control laws, simulation, and practical results of the ball and beam system. Section 5 concludes the overall efforts being made in this study. In the end more relevant recent articles are enlisted.

#### 2. Problem Formulation

The dynamic equations which govern the motion of the class of underactuated system can be presented aswhere , , and are -dimensional position, velocity, and acceleration vectors and , , , and represent the inertia, Coriolis, gravitational, and fractional torques matrices, respectively. is the measured control input, and represents the uncertainties in the control input channel whereas is the control input channel.

It is assumed that and the origin is considered to be the equilibrium point for the aforementioned system. Now, the system in (1) can be rewritten in alternate form as follows:where represents the states of the system and and point to the states. In order to design a control law, the system in (2) can be transformed into two formats which are described in the subsequent study.

##### 2.1. System in Cascaded Form

Following some algebraic manipulations, the system in (2) may be written in cascaded form as follows [17]:where are measurable states of the systems such that and are pointing to the position and velocity of the indirect actuated system (3) while and represent the position and velocity of the directly actuated system (4). represents the controlled signal, as already discussed, to the system (4) input. Owing to the assumption stated immediately after (1), the inverse of exists. The nonlinear functions , are smooth in nature. Now, following the procedure of [6], the disturbances are deliberately introduced to get an approximate controllable canonical form. Note that practical systems like inverted pendulum [18], TORA [4], VTOL (vertical take-off and landing) aircraft [17], and quad rotor [19] can be put in the form presented in (3) and (4). Before proceeding to the control design of the above cascaded form, the following assumptions are made.

*Assumption 1. *Assume thatThis condition is necessary for the system origin to be in equilibrium point when the system is operated in closed loop.

*Assumption 2. * is invertible or is invertible.

*Assumption 3. * is an asymptotically stable manifold, that is, , and approaches zero.

Note that Assumptions 2 and 3 lie in the category of nonnecessary conditions. These are only used when one needs to furnish the closed loop system with a sliding mode controller (see for details [6]).

##### 2.2. Input Output Form

The system in (3) and (4) can be transformed into the following input output form while following the procedure reported in [16]. Let us assume that the system has a nonlinear output . To this end we denote Recursively, it can be written asAssume that the system reported in (3)-(4) has a relative degree “” with respect to the defined nonlinear output. Therefore, owing to [20], one hassubject to the following conditions:(1), where indicates the neighborhood of for;(2), where represents the matched unmodeled uncertainties. System (8), by defining the transformation [21], can be put in the following form: where the transformed states are phase variables, is the control input, and represents matched uncertainties. It is worthy to notice that the inverted pendulum and the ball and beam systems can be replaced in the aforementioned form.

Note that both the formats are ready to design the control law for these systems. In the next section, we outline the design procedure for both the forms.

#### 3. Control Law Design

The control design for the forms presented in (3)-(4) and (9) is carried out in this section which we claim as our main contribution in this paper. The main objective in this work is to enhance the robustness of the system from the very beginning of the process which is the beauty of integral sliding mode control. In general, the integral sliding mode control law appears as follows [14]. In the subsequent subsections, the authors aim to present the design procedure.

##### 3.1. Integral Sliding Mode

This variant of sliding mode possesses the main features of the sliding mode like robustness and the existence chattering across the switching manifold. On the other hand, the sliding mode occurs from the very start which, consequently, provides insensitivity of disturbance from the beginning. The control law can be expressed as follows:where the first component on the right hand side of (10) governs the systems dynamics in sliding modes whereas the second component compensates the matched disturbances. Now, the aim is to present the design of the aforesaid control components.

###### 3.1.1. Control Design for Case-1

This control design for case-1 is the main obstacle in this subsection. To define both the components, the following terms are defined:Using these new variables, the components of the controller are designed in the following subsection. For the sake of completeness the design of this component is worked out via simple pole placement. Following the design procedure of pole placement method, one getswhere are the gains of this control component. This control component steers the states of the nominal system to their defined equilibrium. Now, in the subsequent study the design of the uncertainties compensating term is presented. An integral manifold is defined as follows:where represents the conventional sliding manifold which is Hurwitz by definition.

Now, computing along (3)-(4), one hasNow, choose the dynamics of the integral term as follows:The expression of the term which compensates the uncertainties may be written as follows:The overall controller will look likeThe constants ’s are control gains which are selected intelligently according to bounds. In the forthcoming paragraph, the stability of the presented integral sliding mode is carried out in the presence of the disturbances and uncertainties. Consider the following Lyapunov candidate function:The time derivative of this function along dynamics (11) becomesThe substitution of (15)-(16) results in the following form:subject to .

This expression confirms the enforcement of the sliding mode from the very beginning of the process, that is, in finite time. Now, we proceed to the actual system’s stability. If one considers as the output of the system, then , , and become the successive derivatives of . Whenever is achieved, the dynamics of the transformed system (11) will converge asymptotically to zero under the action of the control component (12) [22]. That is, in closed loop, the transformed system dynamics will be operated under (12) as follows:and the disturbances will be compensated via (16).

The asymptotic convergence of , , , and to zero means the convergence of the indirectly actuated system (3) to zero. On the other hand, the states of the directly actuated system (4) will remain bounded; that is, state of (4) will have some nonzero value in order to keep at zero. Thus, the overall system is stabilized and the desired control objective is achieved.

##### 3.2. Control Design for Case-2

The nominal system related to (9) can be replaced in the subsequent alternative formwhere . It is assumed that at in addition to the next supposition that (22) is governed by :orwhereOnce again, following the pole placement procedure, one may have, for the sake of simplicity, the input which is designed via pole placement, that is,Now to get the desired robust performance, the following sliding manifold of integral type [14] is defined:where is the usual sliding surface and is the integral term. The time derivative of (27) along (9) yieldsThis control law enforces sliding mode along the sliding manifold defined in (27). The constant can be selected according to the subsequent stability analysis.

Thus, the final control law becomes

Theorem 4. *Consider that are satisfied; then the sliding mode against the switching manifold can be ensured and one haswhere is a positive constant.*

*Proof. *To prove that the sliding mode can be enforced in finite time, differentiating (22) along the dynamics of (3)-(4), and then substituting (30), one hasSubstituting (28) in (32), and then rearranging, one obtainsNow, the time derivative of the Lyapunov candidate function , with the use of the bounds of the uncertainties, becomesThis expression may also be written asprovided thatThe inequality in (35) presents that approaches zero in a finite time [23], such thatwhich completes the proof.

#### 4. Illustrative Example

The control algorithms presented in Section 3 are applied to the control design of a ball and beam system. The assessment of the proposed controller, for the ball and beam system, is carried out on the basis of output tracking, robustness enhancement via the elimination of reaching phase, and chattering-free control input in the presence of uncertainties.

##### 4.1. Description of the Ball and Beam System

The ball and beam system is a very sound candidate of the class of underactuated nonlinear system. It is famous because of its nonlinear nature and due to its wide range of applications in the existing era like passenger cabin balancing in luxury cars, balancing of liquid fuel in vertical take-off objects. In terms of control scenarios, it is an ill-defined relative degree system which, to some extent, does not support input output linearization. A schematic diagram with their typical parameters of the ball and beam system is displayed in the adjacent Figure 1 and Table 1, respectively. In this study the authors use the equipment manufacture by GoogolTech. In general this system is equipped with a metallic ball, which is let free to roll on a rod having a specified length, having one end fixed and the other end moved up and down via an electric servomotor. The position of the ball can be measured via different techniques. The measured position is used as feedback to the system and accordingly the motor moves the beam to balance the ball at user defined location.