Research On Several Problems On Flight Control Of Coaxial Rotor/ducted-fan Unmanned Helicopter

The dissertation focuses on the control design of an advanced prototype unmanned helicopter with novel structural and aerodynamic configuration. In particular unmanned helicopter with coaxial rotor and ducted fan configuration is designed and developed that has the advantage of higher aerodynamic efficiency, stronger gust attenuation performance and more compact structure as to conventional helicopters. Thus such helicopter has very wide range of potential applications. Unique structure, aerodynamic configuration and flight mechanism leads to significantly different flight dynamics for the prototype helicopter. Such systems are very challenging from unmanned helicopter control design point of view. The dissertation therefore covers research based on practical problems emerging from the work on the prototype unmanned helicopter, such as analysis of flight dynamics, flight controller design, hover pendulum oscillation attenuation, deal with uncertainties, and multi-objective requirements, etc.First, current research on unmanned helicopters and existing flight control techniques are reviewed and some critical problems in the development of the unmanned helicopter are investigated. The characteristics of the prototype configuration and flight dynamics are derived. After analyzing the dynamic model the prototype unmanned helicopter a linearized model is derived. Based on the linearized model, stability and operation characteristics of the prototype unmanned helicopter is analyzed based on aerodynamic derivatives, motion mode and time-domain responses, and the results are compared with a conventional helicopter. Analysis results show that the prototype unmanned helicopter has unique flight dynamics, such as special hover pendulum oscillation motion mode, inter-axes couplings characteristics and strong anti-wind disturbance capability.Next, based on the flight control requirement of the unmanned helicopter, attitude, velocity and position control laws are designed. Closed-loop model of helicopter in hover is then derived. Problem of hover pendulum-like oscillation motion of closed-loop helicopter is introduced. In order to reveal the causal mechanism of helicopter hover pendulum oscillation, phenomena of physical pendulum motion is investigated and control strategy for pendulum control is proposed. Based on the similarity between physical pendulum and hovering unmanned helicopter in structure, response characteristics and motion mechanism, hover pendulum control strategy is proposed for prototype unmanned helicopter by controlling the movement of main rotor hub is proposed. The decoupled attitude control loop is used as inner loop, pendulum oscillation elimination controllers is then designed for longitudinal and lateral channel separately based on linear quadratic optimal control technique.Considering various uncertainties in unmanned helicopter systems, robust control approaches, suitable for designing unmanned helicopter flight control systems, are investigated. Uncertainties in unmanned helicopter systems are formulated as interval variations in stability and control derivatives. Taken in to consideration the effect of steady and unsteady gust disturbances, two robust flight control strategies are proposed, respectively. One is referred to as robust tracking decoupling control method based on reference command tracking control structure and the other is multi-mode control method based on multi-loop control structure (combining robust H-infinity and PI control). Robust flight controllers are designed for unmanned helicopters based on the proposed methods. Analysis and simulation results show that Level 1 handing requirements, as defined in ADS-33E, are accomplished.Next, problem of control design for uncertain systems with multiple control objectives is investigated. Multiple control objectives for closed-loop system are defined such as robustness, fragility, disturbance attenuation performance and dynamical response characteristics. The problem of robust and non-fragile H-infinity control with regional pole constraint in a disk is investigated, for a class of uncertain systems with norm bounded parametric uncertainties. Disturbance attenuation performance is evaluated by L2 gain from energy-bounded disturbance input to regulated output signal. Dynamical response characteristics are improved by placing closed-loop poles in a prescribed disk region in the complex plane. Two classes of controller perturbations are considered, i.e., additive and multiplicative. Solvability condition for the existence of robust and non-fragile H-infinity controller is derived in the form of a linear matrix inequality (LMI). An illustrative numerical example and an unmanned helicopter control example are utilized respectively to demonstrate the design procedure. Analytical results show the validity and effectiveness of the proposed design method.