## 3 Design of the Electro-Hydraulic Servo Loading System  
The design requirements of the electro-hydraulic servo loading system are shown in Table 1.  
The electro-hydraulic servo loading system is mainly composed of a controller, a servo amplifier, an electro-hydraulic servo valve, a hydraulic cylinder, and a force sensor, as shown in Figure 2. The control goal of the system is to ensure the output force of the hydraulic cylinder tracks the given signal quickly and accurately. The specific working process of the electro-hydraulic servo loading system is as follows: the force sensor detects the output force of the hydraulic cylinder and converts it into a voltage signal, which is compared with the voltage of the given signal to obtain the deviation; the controller calculates the control voltage value according to the deviation and then converts it into the corresponding current signal through the servo amplifier; the current signal controls the movement of the electro-hydraulic servo valve and then drives the hydraulic cylinder; the force sensor detects the output force of the hydraulic cylinder for feedback comparison, and the cycle is repeated to achieve precise control of the output force of the hydraulic cylinder. The PID control controls the system by combining the proportion, integral, and differential of the error signal; because of its simple structure, convenient implementation, good adaptability, and its wide use in engineering practice, the test device initially adopted the PID controller. \begin{table}
\begin{tabular}{c c} \hline \hline Loading waveform &
\begin{tabular}{c} Customizable static and dynamic \\ loading waveforms \\ \end{tabular} \\ \hline Loading amplitude & 0 kN–50 kN \\ \hline Loading direction & Vertical direction \\ \hline Dynamic loading frequency range & 0 Hz–15 Hz \\ \hline Loading amplitude accuracy & 0.5 kN \\ \hline \hline \end{tabular}
\end{table} 
Table 1: System design requirements. The schematic diagram of the hydraulic system of the electro-hydraulic servo loading system is shown in Figure 3. The system uses a fixed displacement vane pump as the power source and is driven by a three-phase asynchronous motor. The variation range in the oil supply pressure is controlled by the pressure relay and the unloading valve: when the system pressure rises to a certain value, the pressure relay sends a signal to prompt the unloading valve to unload the hydraulic pump, while the system pressure is maintained by the accumulator. When the system pressure drops to a certain value, the pressure relay sends a reverse signal to change the unloading valve to a loaded state, the hydraulic pump supplies oil to the system, and it fills the accumulator at the same time. The accumulator is used for energy storage and oil supply, reducing system noise, absorbing the energy generated when the system pressure changes suddenly, and improving the stability of the system. Check valves are used to prevent the reverse flow of oil. The electro-hydraulic servo valve is a key component in the system, which connects the electrical part of the system to the hydraulic part to enable the conversion and amplification of the electrical and hydraulic signals and control of the hydraulic cylinder. The system used MOOG's dual-nozzle-baffle two-stage flow servo valve G761-3005B. The test device has two sets of loading devices, one using an electro-hydraulic servo loading system and the other using an electro-hydraulic proportional loading system, whose principle is similar to that of the electro-hydraulic servo loading system, used to simulate static loads with higher load values. In this work, the electro-hydraulic servo loading system was studied. The forced structure includes the deflection or rebound modulus loading tooling, which is applied to the deflection and rebound modulus detection, respectively. Through tooling, the applied load is transferred to the subgrade or pavement.