# Load Prediction Control Study of a Pitch Control System for Large Offshore Wind Turbines
## 4 Experimental and Simulation Results
### Experimentation  
The electro-hydraulic servo pump-controlled independent variable pitch experimental system is shown in Figure 14. The electrical system includes programmable controllers, servo drives, servo motors, sensors, and other components. The servo motor drives the hydraulic pump coaxially, and the hydraulic cylinder position control is realized by adjusting the flow output of the hydraulic pump; the use of the oil charge accumulator with the liquid-controlled check valve compensates for the asymmetric flow and the internal and external leakage flow of the system; the use of the paddle accumulator enables the system to complete the paddle action and adjusts the paddle pitch angular speed when padding via the unidirectional throttling speed control valve. When the system is overloaded, the system will be unloaded via the relief valve. The controller can monitor and collect the pressure, position, temperature, and other state parameters in the system in real time through the sensor, and through the designed position control strategy, based on load perturbation  
Figure 12: Random disturbing forces. Figure 13: Hydraulic cylinder displacement under random disturbance force. compensation, according to the target displacement and the actual displacement after the comparison of the output servo motor speed signals, the use of EtherCAT communication will provide the desired speed output to the servo drive, followed by the drive through the encoder cable to the servo motor speed control, controlling the hydraulic pump output flow to realize the position of the hydraulic cylinder. The maximum output flow rate of the motor pump set used in this experiment is 118 L/min, the maximum pressure is 35 MPa, the maximum stroke of the hydraulic cylinder used is 400 mm, and the other key components used are shown in Table 3.  
Setting the displacement step signal of the hydraulic cylinder from 100-700 mm, its simulation results in PID control and LDC control are shown in Figure 15. The time required to reach the steady state under PID control is 18.2 s, and the time required to reach the warm pond under LDC control is 17.4 s.  
The position control accuracy and tracking ability of the designed load disturbance compensation controller are mainly tested via continuous step and sinusoidal response tests, and the test results are shown in Figures 16 and 17. From Figure 16, the average time required to reach a steady state for each segment is 4.97 s for PID control and 4.38 s for LDC control; it can be seen that the designed LDC control method is able to realize fast and overshoot-free control of continuous step commands for hydraulic cylinders, which meets the demand of continuous pitch control for variable pitch system. Figure 14: Electro-hydraulic servo pump-controlled independent variable pitch experimental system. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component Name** & **Model** & **Manufacturer** \\ \hline Accumulator & NXQ-AB-25 & Liming (Zhoushan, China) \\ Check Valve & S20P21B & Huade (Beijing, China) \\ Relief Valve & RDFALWN & SUN (Houston, TX, USA) \\ Reversing Valve & DFEBMCN-224 & SUN \\ Throttle Valve & NFDDLGN & SUN \\ Temperature Sensor & PH4000 & Zhuhong (Wuxi, China) \\ Pressure Sensors & PH3600 & Zhuhong \\ Displacement Sensor & SSI Displacement Sensors & Xiju (Nanjing, China) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Key experimental components and their manufacturers. A sinusoidal signal with an amplitude of 50 mm and a gradually increasing frequency from 0.05 to 0.08 Hz was applied; the results are shown in Figure 17. The displacement sensor used has a resolution of 50 \(\upmu\)m and a sampling frequency of 40 Hz. The designed LDC control method is able to track the sinusoidal signal quickly and without amplitude decay. In addition, compared with the PID control curve, the maximum tracking error of the PID control is 12.66 mm, the maximum tracking error of the LDC control is 2.42 mm, and the displacement variance between the PID and the target signal is 43.18.