Modified third‐order super‐twisting controller for chattering mitigation in the digital implementation of a multicopter attitude control

Abstract

This paper presents the design and digital implementation of a controller for Q1 the attitude (orientation) of an eVTOL vehicle that must meet high precision requirements. To fulfil this objective, the closed-loop attitude system has to function as the actuator for the multicopter's position control system, which calls for fast closed-loop attitude dynamics and robust performance. Sliding mode super-twisting-based controllers (STC) have demonstrated effectiveness in handling such type of requirements in manifold applications, positioning this technique as a promising control approach. Therefore, as a first goal, this work assesses the performance of STC for the attitude control challenge and establishes their suitability for this specific application. Furthermore, it is determined that a controller based on the third-order super-twisting algorithm (3-STC) is particularly well-suited for this task. Once the 3-STC is selected, the work progresses to tackle the challenges of its implementation. It is found that when deployed in a realistic multicopter environment, the use of a standard fixed-gain STC would lead to significant chattering, imposing severe limitations on the design of the attitude controller gains. Then, the main goal of the paper is to effectively address this issue when dealing with practical digital implementation. Different countermeasures are proposed in this work to solve such problem; particularly, a modified 3-STC super-twisting law is developed. It turns into a linear proportional–integral (PI) inside a prescribed neighborhood of the commutation variable, while beyond this region, the gains are increased, in accordance with a barrier function. The proposed controller is validated through high-fidelity simulations within a software-in-the-loop (SIL) environment designed to test flight navigation and control firmware.