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We present in this paper the modeling and flight-control-system design for the yaw channel of an unmanned-aerial-vehicle (UAV) helicopter using a newly developed composite nonlinear feedback (CNF)-control technique. The CNF-control method has been proven to be capable of yielding a fast transient response with no or very minimal overshoot in tracking a specific target. From the actual flight tests on our UAV helicopter, it has been found that the commonly used yaw dynamical model for the UAV helicopter proposed in the literature is very rough and inaccurate, which might cause the helicopter to shake severely in certain flight conditions. This motivates us to first obtain a more accurate model for the yaw channel of our UAV helicopter. The CNF-control method is then utilized to design an efficient control law, which gives excellent overall performance. In particular, our design has achieved a Level I performance according to the standards set for military rotorcraft. The results are verified through actual flight tests. Index Terms--Aircraft control, modeling, nonlinear control, unmanned-aerial-vehicle (UAV) helicopters.

We adopt in this paper the newly developed composite nonlinear feedback (CNF) control method to track general target references for systems with input saturation. The original formulation of the CNF control technique is only applicable to set-point tracking, in which the target reference is set to be a constant. In this paper, a reference generator, which is able to produce more general reference signals such as sinusoidal and other waves, will be proposed to supplement the CNF control technique to yield a good performance. The resulting control law comprises the reference generator and a modified CNF control law, which is proven to be capable of tracking a target reference with fast settling time and minimal overshoot. Simulation and experimental results on an XY-table show that the proposed technique gives a very satisfactory performance. Index Terms--Actuator saturation, control applications, motion control, nonlinearities, robust control, servo systems, tracking control.

We study in this paper the theory and applications of a nonlinear control technique, i.e., the so-called composite nonlinear feedback control, for a class of linear systems with actuator nonlinearities. It consists of a linear feedback law and a nonlinear feedback law without any switching element. The linear feedback part is designed to yield a closed-loop system with a small damping ratio for a quick response, while at the same time not exceeding the actuator limits for the desired command input levels. The nonlinear feedback law is used to increase the damping ratio of the closed-loop system as the system output approaches the target reference to reduce the overshoot caused by the linear part. It is shown that the proposed technique is capable of beating the well-known time-optimal control in the asymptotic tracking situations. The application of such a new technique to an actual hard disk drive servo system shows that it outperforms the conventional method by more than 30%. The technique can be applied to design servo systems that deal with "point-and-shoot" fast targeting. Index Terms--Actuator saturation, control applications, hard disk drives, nonlinear control, servo systems.

In this paper, we consider the problem of designing a robust controller for a multivariable servomechanism of a free gyro-stabilized mirror system where there exists cross-coupling between the axes. The nonlinear dynamics of the system are first linearized and reformulated into an [H.sub.[infinity]] problem. A reduced order output feedback controller is then designed using the asymptotic timescale and eigenstructure assignment (ATEA) technique. The overall design has been implemented using a personal computer with C++ and tested on the actual gyro-stabilized mirror system. Our simulation and implementation results show that the design is very successful.