# PSO Optimized Active Disturbance Rejection Control for Aircraft Anti-Skid Braking System
## 1 Introduction  
The aircraft anti-skid braking system (ABS) is an important aircraft subsystem to ensure the safe landing of the aircraft. With the development of the aviation industry, the aircraft is developing in the direction of high speed and large tonnage, using stable and efficient anti-skid braking control. The ABS strategy is of great significance to improve the ground safety of the aircraft [1]. The braking process of the aircraft is a process with strong nonlinearity, including changes in runway conditions, changes in the braking environment, and so on [2], and the aircraft landing time is very short. According to domestic and foreign statistical data, accidents in the landing and braking stage account for as high as 49.1% of all aircraft safety accidents [3]. This poses a serious challenge to the design of aircraft anti-skid braking systems. The anti-skid braking device (ABS) performs a crucial function in the aircraft landing system. In fact, ABS can noticeably enhance the protection of an aircraft in intense circumstances, as it maximizes the longitudinal tire/road friction, while preserving the massive lateral forces that support aircraft steerability [3]. One of the essential challenges in designing ABS is to deal with two sizeable sources of uncertainty affecting the overall landing dynamics, that is, the surprisingly nonlinear tire/road friction forces and the dynamic load switch between the front and rear axle [4]. Many efforts have been committed to adjusting the control of the slip ratio and regulating the reference deceleration rate to achieve proper monitoring of the best adhesioncoefficient of ABS [5]. However, some inherent characteristics, such as complicated nonlinearity and steady stochastic disturbance, make progress in the design of ABS difficult. For a strong real-time and highly nonlinear system such as ABS, the unmodeled dynamics and parametric uncertainty may lead to serious performance degradation [6]. Therefore, model-free control technology has gradually attracted the attention of anti-skid system researchers. The model-free control techniques are also known as data-driven techniques [7], because a large amount of test data is needed to design the controller. Currently, a pressure-bias-modulated (PBM) aircraft anti-skid braking system is the most commonly used braking system, which is an improved algorithm of PID control. It retains the advantages of a simple and easy-to-understand PID controller and does not require an accurate system model of the controlled object. Although a large number of new anti-skid control algorithms have emerged in recent years, it is still the most widely used and installed anti-skid control algorithm. Active disturbance rejection control technology (ADRC) is a new practical technology that adapts to the trend of digital control, absorbs the achievements of modern control theory, develops and enriches the essence of PID thought, and develops and utilizes special nonlinear effects. Therefore, in all occasions where conventional PID can be used, as long as it can be digitized, the use of ADRC will fundamentally improve its control quality and control accuracy. Especially in occasions where high-speed and high-precision control is required in harsh environments, ADRC technology is superior. ADRC is a fairly high-quality method for the manipulation and disturbance rejection of nonlinear systems. This methodology has obtained massive interest in the literature. The philosophy underlying ADRC was outlined by [8; 9]. More recently, the methodology was introduced in English through a series of scientific papers [10; 11; 12]. The main problems encountered in the study of aircraft anti-skid braking are the complexity of the braking system and the uncertainty of braking conditions. Domestic brake system designs mainly adopt a hydraulic control system, and the algorithm is mainly based on the multi-threshold PID+PBM control algorithm of the slip speed control type.