IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2015 231 Guidance of Air Vehicles: A Sliding Mode Approach Muhammad Zamurad Shah, Student Member, IEEE, Raza Samar, Member, IEEE, and Aamer Iqbal Bhatti, Senior Member, IEEE Abstract—This paper presents a novel nonlinear guidance scheme for ground track control of aerial vehicles. The pro- posed guidance logic is derived using the sliding mode con- trol technique, and is particularly suited for unmanned aerial vehicle (UAV) applications. The main objective of the guidance algorithm is to control the lateral track error of the vehicle during flight, and to keep it as small as possible. This is achieved by banking the vehicle, that is, by executing roll maneuvers. The guidance scheme must perform well both for small and large lateral track errors, without saturating the roll angle of the vehicle, which serves as the control input for the guidance algorithm. The limitations of a linear sliding surface for lateral guidance are indicated; a nonlinear sliding surface is thereafter proposed which overcomes these limitations, and also meets the criterion of a good helmsman. Stability of the nonlinear surface is proved using Lyapunov theory; control boundedness is also proved to ensure that the controls are not saturated even for large track errors. The proposed guidance law is implemented on the flight control computer of a scaled YAK-54 UAV and flight results for different scenarios (consisting of both small and large errors) are presented and discussed. The flight test results confirm the effectiveness and robustness of the proposed guidance scheme. Index Terms—Control boundedness, guidance and control of unmanned aerial vehicles (UAVs), lateral guidance, sliding mode control (SMC), track control, UAVs. I. I NTRODUCTION U NMANNED AERIAL VEHICLES (UAVs) are of great interest for intelligence, surveillance and reconnaissance applications, and also for rescue operations. They reduce risk to life, and have a relatively low operational cost. In recent years, the work on UAV autonomy has added a new dimension to the utility of these vehicles. Autonomy is the ability to perform a task (mission) without being directly or remotely controlled by a human operator [1]. Autonomous vehicles must have sufficiently advanced path planning algorithms, combined with effective and robust guidance and automatic control systems. Successful control system design for high- performance UAVs requires efficient and effective techniques for the design of guidance and control algorithms that ensure satisfactory operation in the face of system uncertainties and Manuscript received April 16, 2013; revised November 26, 2013; accepted May 1, 2014. Date of publication June 3, 2014; date of current version December 15, 2014. Manuscript received in final form May 4, 2014. Rec- ommended by Associate Editor A. Behal. The authors are with the Department of Electronics Engineering, Moham- mad Ali Jinnah University, Islamabad 44000, Pakistan (e-mail: zamurad@ gmail.com; raza.samar@gmail.com; aamer987@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCST.2014.2322773 environmental disturbances. A key performance criterion for the guidance and control system is to have the ability of precise ground track control in the presence of disturbing forces. A criterion for assessing the lateral performance and handling qualities for such vehicles is presented in [2]. Two approaches are usually employed for ground track con- trol of UAVs. In the first approach, the guidance and control design problems are separated into an outer-loop guidance and an inner-loop control design problem [3]–[7]. In the second approach, the guidance and control problems are addressed together in an integrated and unified framework [7]–[11]. A good background to the integrated approach and associated challenges is presented in [12] and [13]. This approach is more complicated due to the coupling of the different guidance and control variables, and has not been as popular among the practising aerospace community. In most applications, the first approach is employed owing to its simplicity and intu- itive appeal. Various well-established techniques exist for the (inner loop) control design problem, such as linear, nonlinear, robust, and intelligent techniques [14]. Work has also been done on the design of the guidance loop; methods such as proportional navigation, vector field methods, vision-based methods and neural networks have been used, see for example [9], [15]–[17]. A lateral track control law for small UAVs has been dis- cussed in [6]; this is based on a pure geometrical concept. The idea is to make the ratio of lateral deviation to lateral velocity equal to the ratio of longitudinal distance to longitudinal veloc- ity. Simulation results indicate that the yaw-rate command generated by the guidance law exhibits oscillations in the vehi- cle roll channel, this could be a problem for implementation on a real vehicle. Mixed integer linear programming-based guidance for UAVs has also been considered [18], [19]; how- ever, here the optimization program generates a sequence of waypoints (positions) and velocities for the vehicle to follow. In other words, a mission plan is generated and deviations from this plan need to be corrected through a lower level guidance algorithm. A conceptually different guidance scheme employing vector fields for curved path following has also been pursued, see for example [20], [21]. Here a vector field of course commands is generated, which is a function of vehicle position relative to the desired track. The difference between the actual and commanded course angles forms the error which is driven to zero by using an appropriate control algorithm. The vector field can, in some cases, give rise to large and sudden course commands, which can tax the capability of the control system. 1063-6536 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.