## 1 Introduction  
Pavement mechanical properties include the deflection and rebound modulus, which reflect the state characteristics of the intensity, stiffness, bearing capacity, and stability of the pavement structure [1, 2]. At present, the commonly used measurement methods for the deflection include the Benkelman beam, auto deflectometer, and falling weight deflectometer (FWD) methods [3, 4]. Commonly used measurement methods for the rebound modulus include the Benkelman beam, bearing plate, FWD, and portable falling weight deflectometer (PFWD) methods, etc. [5, 6, 7, 8]. During the measurement, a simulated load is applied to the pavement, provided by a load-generating device, to simulate the actual force. The Benkelman beam method uses a truck with a standard axle load on the rear axle as the load-generating device, and the required load is achieved by counterweighting the truck. The auto deflectometer method uses an integrated carrier car with a rear-axle standard axle load of 10 tons as the load-generating device. The load of the FWD and PFWD methods is provided by a heavy hammer, and the impact load is changed by changing the hammer's mass and height. The bearing plate method uses a hydraulic jack to provide the load, and the load is changed by adjusting the hydraulic jack. There are problems with the load-generating devices of these measurement methods: either only a static or dynamic load can be simulated, the simulated load type is single, the amplitude is limited, the loading accuracy is low, the loading speed is slow, and the loading steps are cumbersome. An electro-hydraulic servo system has the merits of high control accuracy, a fast response speed, large output power, and flexible signal processing, and it is easy to determine the feedback of various parameters [9]. In a typical electro-hydraulic servo system, the regulated variable is the force or torque. At present, electro-hydraulic servo loading systems have been widely used in the reinforced concrete towers of large wind turbines, triaxial apparatus, helicopter-manipulating boosters, pumping units, and other fields [10; 11; 12; 13]. An electro-hydraulic servo system has characteristics of nonlinearity, time variation, parameter uncertainty, and mechanical-hydraulic coupling [14], which increases the difficulty of system control. In order to achieve a better quality of the system control of an electro-hydraulic servo system, scholars have carried out extensive and in-depth research, introducing different control strategies, such as intelligent, PID, backstep, active disturbance rejection (ADRC), and sliding mode (SMC), which have achieved good control effects. [PERSON] [15] optimized the coefficients of the PID controller using the genetic algorithm, which improved the dynamic response characteristics of the system. [PERSON] et al. [16] proposed an improved particle swarm optimization algorithm to optimize the coefficients of the PID controller, and the system trajectory tracking accuracy was improved. [PERSON] et al. [17] proposed an optimal robust adaptive fuzzy backstepping control method, which suppressed the chattering problem and effectively reduced the disturbances, while ensuring the system performance was tracked. [PERSON] et al. [18] proposed a backstepping control scheme based on the differentiator by using finite-time-convergent second-order differentiators, which reduced the position tracking error of the system. ADRC regards the unmodeled dynamics, nonlinearity, disturbance, etc., of the system as the 'total disturbance' and estimates it through ESO, to carry out targeted dynamic compensation and optimize system performance [19]. [PERSON] [20] proposed a linear ADRC form, which simplified the controller parameter setting and promoted the development of ADRC. In recent years, ADRC has been applied increasingly in the field of electro-hydraulic servo systems. [PERSON] et al.