## 4 Application of the Automatic Assembly Production Line
### Quality Improvement of the Preload Loading Process  
Taking proportional solenoid valves as an example, the six performance indicators of valve startup voltage, staminrtup displacement, stroke, compression, steady-state voltage, and linearity of the displacement curve need to meet the requirements simultaneously. The valve core motion structure adopts a suspension structure of sheet springs to achieve frictionless motion, and the loading force will affect the valve performance.  
The valve core of electric valve products is designed as a suspension structure of disc springs to achieve frictionless operation. If the disc springs are fixed too loosely or too tightly, it will affect the response performance of the valve. In the past, manual assembly processes used fixtures to fix and control the tightening torque of screws for indirect force loading, resulting in large errors and uneven compression forces of multiple screws. For valve products that are sensitive to loading force, such as proportional solenoid valves, multiple disassembly, assembly, and adjustment processes are required during the assembly process. This automatic assembly process uses a dynamometer and multiple motion mechanisms for closed-loop control to complete the force loading process. The impact of loading force on valve performance can be quantified and decoupled from  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Process Steps** & **Manual Assembly Cycle** & **Automatic Assembly Cycle** & **Efficiency** \\ \hline \hline \begin{tabular}{c} Preload loading \\ Measurement of valve spool stroke \\ Running-in, testing \\ Welding of shell structure \\ \end{tabular} & \begin{tabular}{c} 0.5 h \\ 2 h/2 persons \\ 1 h \\ \end{tabular} & \
\begin{tabular}{c} 2 min \\ 2 min/1 person \\ 2 min \\ 2 min \\ 2 min \\ 2 min \\ 2 min \\ 2 50\% \\ \end{tabular} &
\begin{tabular}{c} Improved 92\% \\ Improved 99\% \\ Improved 99\% \\ Improved 30x \\ 20\% \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of single-process operation times between manual assembly and the automatic platform.  
Figure 9: Measurement and testing mechanism.  
various influencing variables. By applying multiple valve products and setting the motion parameters of multiple motion mechanisms reasonably, the accuracy of force loading can reach \(\pm 0.1\) N. An example of a force loading curve of a proportional electromagnetic valve is shown in Figure 10.  
Figure 11 shows the electrical performance curve of the valve with traditional assembly method. The valve is manually tightened, and the pre-tightening force is unknown. Figure 12 shows the electrical performance curve of the valve with automatic assembly method. Its preload can be precisely controlled.  
Figure 11: Electrical performance curve of the valve with traditional assembly method.  
Figure 10: Force loading curve of a proportional electromagnetic valve.  
As shown from the above figures, the loading force is precisely controlled by the automatic assembly method; the electrical performance of the valve is more stable, and there is no current fluctuation during the opening process of the valve.  
The preload will affect the linearity of the valve opening displacement curve. The traditional manual assembly method cannot accurately control the preload, and each product needs to rely on experience to repeatedly disassemble and adjust. The displacement curve with poor linearity is shown in Figure 13. The linearity of valve opening displacement is significantly improved by precise control of loading force, as shown in Figure 14.