AN AUTOPILOT DESIGN FOR THE LONGITUDINAL DYNAMICS OF A LOW-SPEED EXPERIMENTAL UAV USING TWO-TIME-SCALE CASCADE DECOMPOSITION

Abstract

This paper discusses a cascade decomposition method for two-time-scale systems. We decompose the two-time-scale system into slow and fast subsystems connected in cascade form. With the proposed decomposition method, fast and slow low-order subsystems can be easily extracted from the two-time-scale full order system. The longitudinal dynamics of a low speed experimental UAV is used to illustrate the proposed method. An altitude hold control structure is constructed to allow sequential designs based on the lower order fast and slow models using classical designs. The structure contains two control loops. The inner loop is for altitude rate regulation. In this control loop, we use combination of altitude rate and pitch angle as the feedback signals. A fast controller is designed to satisfy the high frequency requirements using the fast model. Following the results of the fast design, three control gains are designed to satisfy the low frequency requirements using the slow models. The design results of the inner loop are used to design the outer altitude control loop. Frequency domain analysis and computer simulations confirm the success of the proposed decomposition method and verify the effectiveness of the control law using the proposed control structure.