Unmanned Rotorcraft Systems

In this monograph, the authors aim to explore the research and development of fully functional miniature unmanned-aerial-vehicle (UAV) rotorcraft, which consist of a small-scale basic rotorcraft with all necessary accessories onboard and a ground station. The unmanned system is an integration of advanced technologies developed in the communications, computing, and control areas. It is an excellent test bed for testing and implementing modern control techniques. It is, however, a highly challenging process. The flight dynamics of small-scale rotorcraft such as a hobby helicopter is similar to its full-scale counterpart but owns some unique characteristics such as the utilization of a stabilizer bar, higher rotor stiffness, and yaw rate feedback control. Besides these, the strict limitation on payload also increases the difficulty in upgrading a small-scale rotorcraft to a UAV with full capacities. Based on its various characteristics and limitations, a lightweight but effective onboard avionic system with corresponding onboard/ground software should be carefully designed to realize the system identification and automatic flight requirements. These issues will be addressed in detail in this monograph. Research on utilizing the vision-based system for accomplishing ground target tracking and following, cooperative control, and flight formation of multiple unmanned rotorcraft is also highlighted.

In Chap. 2, we summarize several coordinate systems used in our work. More specifically, we are to present the concepts of the geodetic coordinate system, the earth-centered earth-fixed coordinate system, the local north-east-down (NED) coordinate system, the vehicle-carried NED coordinate system, the body coordinate system of an unmanned flying vehicle, and the coordinate transformations among them, which are heavily used in navigation, guidance, and control of aircraft.

Chapter 3 presents a systematic methodology for constructing a fully functional unmanned rotorcraft system, which involves key steps such as virtual design environment selection, selection of hardware components, design and integration of the avionic system, and performance and reliability evaluation. A Raptor 90 SE-based UAV, named SheLion, a twin of HeLion, but with an additional onboard vision system, is used to illustrate the design procedure throughout the entire chapter.

We aim in Chap. 4 to develop a comprehensive software system that can be universally adopted in unmanned aerial systems. The software system developed consists of two main parts, the onboard software subsystem and the ground control station software subsystem. The onboard subsystem provides reliable support for high precision timer and synchronization operations; processes vision images captured; operates hardware components such as navigation sensors, data acquisition boards, and servo systems; logs data in flying processes; communicates with the ground control station, and implements automatic control algorithms. The ground control station subsystem is programmed for data transferring with the onboard system and for monitoring the flight status of the unmanned system.

Chapter 5 aims to introduce an integration of a low-cost inertial attitude and position reference system for mini UAV helicopters by utilizing the well-known extended EKF technique. More specifically, we propose a systematic signal enhancement procedure for measurement sensors adopted in miniature unmanned aerial systems. The procedure yields more accurate measurement of the Euler angles, angular rates, positions, and velocities of the UAV with a self-integrated navigation unit.

In Chap. 6, we present a comprehensive modeling process to obtain a highly accurate nonlinear dynamical model for our unmanned systems, SheLion (also applicable to HeLion). We first derive a minimum-complexity model structure, which covers all the important dynamic features necessary for flight control law design. Based on this structured model, we develop a five-step procedure, a systematic combination of the first-principles and system identification approaches, to determine all the associated model parameters. We then carry out a thorough validation process to verify the fidelity of the flight dynamics model in the wide flight envelope. Finally, we proceed to determine the flight envelope of the obtained flight dynamics model, which is essential before proceeding to conduct flight control law design and flight experiments.

We propose a three-layer automatic flight control system for our unmanned vehicles based on the time scales of the state variables of the helicopter, which consists of the inner loop, the outer loop, and the flight scheduling layers. The inner loop stabilizes the dynamics of the helicopter associated with its angular velocities and Euler angles. The outer loop controls the position of the unmanned system. Lastly, the outmost layer, i.e., the flight scheduling layer, generates the necessary trajectories for predefined flight missions. Chapter 7 presents the design of the inner-loop control law using an H-infinity control technique based on the linearized model obtained in Chap. 6. More specifically, we focus on issues related to design specification selection, problem formulation, flight control law design, and overall performance evaluation. Design specifications for military rotorcraft set for US army aviation are adopted throughout the whole process to guarantee a top level performance.

We utilize a so-called robust and perfect tracking control technique in Chap. 8 to design the outer-loop control law to control the position of the unmanned system, which is capable of achieving much better performance for situations when complicated maneuvers are required. The robust and perfect tracking control technique is to design a controller such that the resulting closed-loop system is asymptotically stable and the controlled output almost perfectly tracks a given reference signal in the presence of any initial conditions and external disturbances. It makes use of all possible information including the system measurement output and the command reference signal together with all its derivatives, if available, for control. Such a unique feature is particularly useful for the outer-loop layer, in which the position reference and its velocity as well as acceleration all can be measured by the onboard avionic system.

In Chap. 9, we present a fairly comprehensive evaluation of the overall flight control system designed in Chaps. 7 and 8 through hardware-in-the-loop simulations and actual flight tests. We aim to evaluate its performance and robustness by a careful selection of mission-task-elements (MTEs) adopted from ADS-33D-PRF, which is set for military rotorcraft by US army aviation. The selected mission-task-elements for test include depart/abort (forward flight), hover, depart/abort (backward flight), hovering turn, vertical maneuver, lateral reposition, turn-to-target, slalom, and pirouette. The results obtained clearly indicate that our design is very successful. The unmanned rotorcraft system is capable of achieving the desired performance in accordance with the military standard under examination.

To conclude the whole monograph, we feature respectively in Chaps. 10 and 11 the applications of the unmanned rotorcraft systems constructed. More specifically, in Chap. 10, we present some basic results on flight formation and collision avoidance of multiple unmanned systems. We adopt the leader-follower pattern to maintain a fixed geometrical formation while navigating the unmanned rotorcraft following certain trajectories. In order to avoid possible collisions in the actual formation flight test, a collision avoidance scheme based on some predefined alert zones and protected zones is employed. Simulations and experimental results are presented to verify our design. We should note that with the utilization of the RPT control technique presented in Chap. 8, the flight formation control is rather straightforward.

Finally, in Chap. 11, we document the design and implementation of a comprehensive vision system for an unmanned rotorcraft to realize missions such as ground target detection and following. To realize the autonomous ground target seeking and following, a sophisticated vision algorithm is proposed to detect the target and estimate relative distance to the target using an onboard color camera together with necessary navigation sensors. The vision feedback is then integrated with the automatic flight control system to guide the unmanned helicopter to follow the ground target inflight. The overall vision system is tested in actual flight missions, and the results obtained show that it is robust and efficient.