Zhixiang Liu

Unmanned surface vehicles: An overview of developments and challenges

Abstract With growing worldwide interest in commercial, scientific, and military issues associated with both oceans and shallow waters, there has been a corresponding growth in demand for the development of unmanned surface vehicles (USVs) with advanced guidance, navigation and control (GNC) capabilities. This paper presents a comprehensive literature review of recent progress in USVs development. The paper first provides an overview of both historical and recent USVs development, along with some fundamental definitions. Next, existing USVs GNC approaches are outlined and classified according to various criteria, such as their applications, methodologies, and challenges. Finally, more general challenges and future directions of USVs towards more practical GNC capabilities are highlighted.

Fault-Tolerant Flight Control Design With Finite-Time Adaptation Under Actuator Stuck Failures

This paper presents an adaptive fault-tolerant flight control (FTFC) scheme to counteract actuator stuck failures, where the issues on saturation avoidance of redundant actuators and finite-time adaptation of FTFC are addressed. The “positive

A Learning-Based Fault Tolerant Tracking Control of an Unmanned Quadrotor Helicopter

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Fault-Tolerant Flight Control Design with Explicit Consideration of Reconfiguration Transients

-modification” technique and the adjustable reference model are exploited for preventing redundant actuators from saturation or secondary damage. In addition, the integration of the auxiliary integrated regressor matrix ensures that the FTFC law is determined within a finite amount of time. Therefore, the accommodation process of actuator stuck failures can be facilitated. Numerical simulation studies based on the nonlinear aircraft models are illustrated to highlight the effectiveness of the proposed methodology.

UAV-based forest fire detection and tracking using image processing techniques

This paper presents a novel learning-based fault tolerant tracking control approach by using an extended Kalman filter (EKF) to optimize a Mamdani fuzzy state-feedback tracking controller. First, a robust state-feedback tracking controller is designed as the baseline controller to guarantee the expected system performance in the fault-free condition. Then, the EKF is employed to regulate the shape of membership functions and rules of fuzzy controller to adapt with the working conditions automatically after the occurrence of actuator faults. Next, based on the modified fuzzy membership functions and rules, the baseline controller is readjusted to properly compensate the adverse effects of actuator faults and asymptotically stabilize the closed-loop system. Finally, in order to verify the effectiveness of the proposed method, several groups of numerical simulations are carried out by comparing the performance of a tracking control scheme and the presented technique. Simulation results demonstrate that the proposed method is effective for optimizing the fuzzy tracking controller on-line and counteracting the side effects of actuator faults, and the control performance is significantly improved as well.

Fault-Tolerant Formation Control of Unmanned Aerial Vehicles in the Presence of Actuator Faults and Obstacles

FAULT-TOLERANT control (FTC) has drawn considerable attention over the last three decades, due to its important role in preventing system breakdown via configured system redundancy [1,2]. FTC designmethodologies can generally be divided into either passive FTC or active FTC [3]. A passive FTC is synthesized under both normal and faulty cases, and a fixed controller is always commissioned, regardless of whether or not prescribed malfunctions are present. On the other hand, an active FTC reacts to specific faults by reconfiguring the controller based on the up-to-date information obtained from a fault detection and diagnosis (FDD) unit. Various design approaches have been exploited for active faulttolerant flight control (FTFC) [1,2,4]. Two notable issues exist in an active FTFC system: 1) The mismatch between the previous and current control signals at the reconfiguration/switching instance usually induces unexpected transients of aircraft outputs [5]. Transients are potentially dangerous to post-failure aircraft, when the healthy actuators attempt to compensate for failed actuators. As pointed out in [1], “However, how to manage or reduce these transients during a controller reconfiguration is still an open issue.” In the latest literature [6], a set of reconfigurable controllers is designed in response to the anticipated fault cases, while a hysteresis supervisory approach is adopted to eliminate switching bumps. 2) Another issue is that healthy actuators are easily saturated due to unexpected transients and improper FTFC design. Saturation of healthy actuators can induce secondary damage to the faulty aircraft and may even jeopardize the safety of the aircraft. In [7–9], the concept of “graceful performance degradation” is applied for the active FTFC design under actuator faults. Healthy actuators’ saturation can be prevented by decreasing the requirements for both transient and steady-state performance. Most of the existing studies address the following notions individually: 1) fault accommodation without explicitly considering the impact of reconfiguration switch actions [1,2], 2) bumpless transfer using dynamic output feedback in fault-free systems [10–12], and 3) controller design within actuator limitations [13–16]. From a safety standpoint, it would be highly desirable if one can design a FTFC scheme to ensure that the process of reconfiguration switching is smooth, while the limits of the remaining actuators are not violated. Motivated by this fact, an adaptation mechanism is developed to prevent abrupt transients and actuator saturation by mitigating the deviation between the aircraft and the target model. Consequently, the active FTFC system, including the reconfigurable controller and the adaptationmechanism, is capable of compensating for the adverse effects resulting from actuator abnormalities and a reconfigurable controller switch. The main contribution is that the proposed FTFC scheme can guarantee a graceful reconfiguration process and respect remaining actuators’ capabilities.

Aerial Images-Based Forest Fire Detection for Firefighting Using Optical Remote Sensing Techniques and Unmanned Aerial Vehicles.

In this paper, an unmanned aerial vehicle (UAV) based forest fire detection and tracking method is proposed. Firstly, a brief illustration of UAV-based forest fire detection and tracking system is presented. Then, a set of forest fire detection and tracking algorithms are developed including median filtering, color space conversion, Otsu threshold segmentation, morphological operations, and blob counter. The basic idea of the proposed method is to adopt the channel “a” in Lab color model to extract fire-pixels by making use of chromatic features of fire. Numerous experimental validations are carried out, and the experimental results show that the proposed methodology can effectively extract the fire pixels and track the fire zone.

Leader-follower formation control of unmanned aerial vehicles with fault tolerant and collision avoidance capabilities

This paper presents a leader-follower type of fault-tolerant formation control (FTFC) methodology with application to multiple unmanned aerial vehicles (UAVs) in the presence of actuator failures and potential collisions. The proposed FTFC scheme consists of both outer-loop and inner-loop controllers. First, a leader-follower control scheme with integration of a collision avoidance mechanism is designed as the outer-loop controller for guaranteeing UAVs to keep the desired formation while avoiding the approaching obstacles. Then, an active fault-tolerant control (FTC) strategy for counteracting the actuator failures and also for preventing the healthy actuators from saturation is synthesized as the inner-loop controller. Finally, a group of numerical simulations are carried out to verify the effectiveness of the proposed approach.

Active Fault-Tolerant Control of Unmanned Quadrotor Helicopter Using Linear Parameter Varying Technique

Due to their fast response capability, low cost and without danger to personnel safety since there is no human pilot on-board, unmanned aerial vehicles (UAVs) with vision-based systems have great potential for monitoring and detecting forest fires. This paper proposes a novel forest fire detection method using both color and motion features for processing images captured from the camera mounted on a UAV which is moving during the whole mission period. First, a color-based fire detection algorithm with light computational demand is designed to extract fire-colored pixels as fire candidate regions by making use of chromatic feature of fire and obtaining fire candidate regions for further analysis. As the pose variations and low-frequency vibrations of UAV cause all objects and background in the images are moving, it is challenging to identify fires defending on a single motion based method. Two types of optical flow algorithms, a classical optical flow algorithm and an optimal mass transport optical flow algorithm, are then combined to compute motion vectors of the fire candidate regions. Fires are thereby expected to be distinguished from other fire analogues based on their motion features. Several groups of experiments are conducted to validate that the proposed method can effectively extract and track fire pixels in aerial video sequences. The good performance is anticipated to significantly improve the accuracy of forest fire detection and reduce false alarm rates without increasing much computation efforts.

Adaptive fault-tolerant control of unmanned quadrotor helicopter using linear parameter varying control technique

In this paper, a leader-follower formation control of multiple unmanned aerial vehicles (UAVs) design methodology is proposed to keep the desired formation, while simultaneously deal with the potential collision and actuator faults. The proposed formation control is divided into outer-loop and inner-loop controllers. First, a leader-follower control structure is constructed as the outer-loop controller. Then, an adaptive fault tolerant control (FTC) scheme along with a collision avoidance strategy are combined as the inner-loop controller. Simulation validations are conducted to demonstrate the effectiveness of this presented design method.