Altitude Control of Quadcopters with Absolute Stability Analysis

Abstract

This paper examines the vertical dynamics of a quadcopter, and presents an approach to achieve better altitude control. The control system includes a multi-sensor-based state observer for estimating vertical speed, a disturbance observer for improving system robustness, and a position controller for tracking the vehicle's altitude with respect to a reference value. This paper clarifies that it is essential to address the model of the propeller actuators to properly guarantee system stability. It was found that the altitude control system is a multi-input multi-output system. By showing the rank-1 interaction between the actuators, this paper derives the condition for the controller that ensures absolute stability of the control system. The condition can be checked conveniently using a graphical test with the Nyquist plot, thereby alleviating the complexity of system design and analysis. The effectiveness of the proposed approach is evaluated by both numerical simulations and real-time experiments. This approach can be easily extended to the motion control of other general multirotor vehicles.