# Optimised Tuning of a PID-Based Flight Controller for a Medium-Scale Rotorcraft
###### Abstract  
This paper presents the parameter optimisation of the flight control system of a single-rotor medium-scale rotorcraft. The six degrees-of-freedom (DOF) nonlinear mathematical model of the rotorcraft is developed. This model is then used to develop proportional-integral-derivative (PID)-based controllers. Since the majority of PID controllers installed in industry are poorly tuned, this paper presents a comparison of the optimised tuning of the flight controller parameters using particle swarm optimisation (PSO), genetic algorithm (GA), ant colony optimisation (ACO) and cuckoo search (CS) optimisation algorithms. The aim is to find the best PID parameters that minimise the specified objective function. Two trim conditions are investigated, i.e., hower and 10 m/s forward flight. The four algorithms performed better than manual tuning of the PID controllers. It was found, through numerical simulation, that the ACO algorithm converges the fastest and finds the best gains for the selected objective function in hower trim conditions. However, for 10 m/s forward flight trim, the GA algorithm was found to be the best. Both the tuned flight controllers managed to reject a gust wind of up to 5 m/s in the lateral axis in hover and in forward flight. rotcraft UAV; optimisation; dynamic modelling; ant colony optimisation; cuckoo search; genetic algorithm; particle swarm optimisation +
Footnote †: journal:  
12 April 2021  
9 May 2021  
SubSubject:3 June 2021  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **Published maps and institutional affiliations. increase in computational power and communication bandwidth has made possible some of the improvements that have eluded control engineers in recent years [5]. As such, a number of control strategies have been presented for the control of rotorcraft, including a proportional-integral-derivative (PID) controller and its gain scheduling counterpart [6; 7]. This design methodology, also referred to as classical, requires linear approximation of the rotorcraft around a selected operating region. This is due to the fact that PID controllers are normally single-input single-output (SISO). However, these methods only work well under the simplifying assumptions of a linear system. Despite this, PID controllers have been successfully implemented in rotorcraft flight control systems. Following the advances in the development of computer systems for flight control, there was a rise in the application of \"modern control\" systems such as the linear quadratic regulator (LQR) [8; 9] and \(H_{\infty}\)[10]. These methods are difficult to implement practically [6]. Other methods, such as nonlinear inverse dynamics [9], including feedback linearisation [11], adaptive control [12] and sliding mode controllers [13], have also been applied with moderate success. The problem with PID has been identified as poor tuning, which means that most of the controllers currently in operation have been poorly tuned. This results in biased judgment against the PID controllers themselves. The best known method for tuning PID controllers is Ziegler-Nichols, based on empirical rules. This method does not work well for multi-loop systems such as rotorcraft. Significant effort has recently been invested in optimal tuning of PID controllers [14; 15; 16] and other controllers in general [17]. [PERSON] et al. [18] applied cuckoo search to optimise PID parameters on a semi-active suspension system with the objective of operating at a desired damping force. For this, they conducted numerical simulations and experimental study on a quarter-car rig and found that the CS-PID does improve ride comfort. [PERSON] et al. [19] investigated the application of GA to optimise a PID controller and a pseudo-derivative controller (PDF) to control a tall building elevator. The optimised controllers showed improved performance when compared to the manually tuned ones, which were not able to meet the settling time requirement.