  Citation: Timis, D.D.; Muresan, C.I.; Dulf, E.-H. Design and Experimental Results of an Adaptive Fractional-Order Controller for a Quadrotor. Fractal Fract. 2022, 6, 204. https://doi.org/10.3390/ fractalfract6040204 Academic Editor: Da-Yan Liu Received: 17 February 2022 Accepted: 4 April 2022 Published: 6 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fractal and fractional Article Design and Experimental Results of an Adaptive Fractional-Order Controller for a Quadrotor Daniel D. Timis, Cristina I. Muresan and Eva-H. Dulf * Department of Automation, Faculty of Automation and Computer Science, Technical University of Cluj-Napoca, Memorandumului Str. 28, 400014 Cluj-Napoca, Romania; daniel.timis@aut.utcluj.ro (D.D.T.); cristina.muresan@aut.utcluj.ro (C.I.M.) * Correspondence: eva.dulf@aut.utcluj.ro Abstract: The use of multi-copter systems started to grow over the last years in various applications. The designed solutions require high stability and maneuverability. To fulfill these specifications, a robust control strategy must be designed and integrated. Focusing on this challenge, this research proposes an adaptive control design applied to a physical model of a quadrotor prototype. The proposed adaptive structure guarantees robustness, control flexibility, and stability to the whole process. The prototype components, structure, and laboratory testing equipment that are used to run the experiments are presented in this paper. The study is focused on the performance comparison of a classical PID controller and a fractional-order controller, which are both integrated into the adaptive scheme. Fractional-order controllers are preferred due to their recognized ability to increase the robustness of the overall closed-loop system. Furthermore, this work covers the design and the tuning method of this control approach. The research concludes with the actual results obtained for this comparative study that highlights the advantages of the fractional-order controller. Keywords: quadrotor design; fractional-order controller; adaptive control structure 1. Introduction The interest to integrate multi-copter systems into different types of aeronautical applications has increased in recent years due to their easy-to-fly readiness and maneu- verability. These systems are widely used for video and photography data gathering in various domains or for aerial surveillance and monetarization of power plants, wind and solar parks, power lines, industrial parks, wildfires, or natural disasters. Search and rescue, refugee localization and first aid assistance represent the most frequent applications of these systems because of their fast response time and easy accessibility. A secondary category of applications is represented by glacial inspection, agriculture crops inspection and spraying, bridges and dam inspection, border patrol, excavation, site documentation, and goods delivery [1,2]. Such aerial vehicles have become widespread in various military and civilian applications. This has led to significant attention in designing advanced control algorithms for such systems in the last few years. This is also the underlining motivation for this research: to evaluate the operation of a quadrotor system using different types of control strategies by performing a series of tests and experiments on a laboratory prototype. A series of testing models exist today [3,4], each with its very own set of advantages and disadvantages. The existing solutions on the market, designed as laboratory test sys- tems, are usually used to simulate a particular performance and are usually expensive. This results in the need to design and build a custom-made laboratory set-up that would allow the implementation of various control strategies, including advanced control algorithms, as well as the experimental testing of these strategies in a wider range of flight scenarios. This represents the motivation behind the design of a custom-made quadrotor within the research lab of the authors. Regardless of the prototype used, the most challenging part Fractal Fract. 2022, 6, 204. https://doi.org/10.3390/fractalfract6040204 https://www.mdpi.com/journal/fractalfract