Design Method for Automatic Assembly Production Line of Electric Valves in Space Propulsion Systems

: This article proposes a design method for a valve automatic assembly production line in response to the automation assembly requirements of electric valve products in space propulsion systems and the engineering problems of inaccurate loading force control and low valve measurement accuracy in existing process methods. This method can achieve ﬁve assembly processes during the assembly process of electric valves, including pre-tightening force control, valve-core stroke measurement, performance testing, and shell structure welding. The article introduces the design of platform components such as process execution, positioning, and transportation, as well as the design and operation process of workstations. By combining the design of a three-axis motion mechanism, a small turntable, and a robotic arm, the product can achieve professional, positioning, full process automation, and equipment miniaturization design across multiple workstations. Through the design of precise control of loading force and non-contact optical measurement method of moving structure, compared with the original method, the parameters affecting product performance are precisely controlled and the precision is improved. And the multivariable decoupling of valve product performance is realized by this method. Through application veriﬁcation, this automatic assembly production line can signiﬁcantly improve the assembly efﬁciency of electric valve products and solve difﬁcult problems in product engineering.


Introduction
Space propulsion systems are important subsystems of various spacecrafts, such as satellites, spacecraft, and probes; their mission is to provide the orbit and attitude control force and torque of the spacecraft [1]. The composition of the propulsion system includes propellant storage units (gas cylinders, tanks), fluid control components (various electric and mechanical valves), and liquid engines [2]. The electric valves in fluid control components account for the largest proportion of the entire propulsion system. They provide precise fluid control for the propulsion system and precise impulse for the liquid engine as an actuator.
The electric valves in space propulsion systems are divided into solenoid valves, selflocking valves, proportional valves, etc., based on their functions. Most of them are solenoid axial flow structures with a compact structure and high accuracy [3,4]. The overall weight of some products is only a few tens of grams. A schematic diagram of the composition and structure of a typical electric valve is shown in Figure 1.
In order to meet the requirements of miniaturization and lightness, and to avoid material wastage during the assembly process, the electric valve components used in the space propulsion system are all of a threaded structure. Finally, the valve parts are welded and fixed via electron beam welding. The unthreaded structure of electric valve products In order to meet the requirements of miniaturization and lightness, and to avoid material wastage during the assembly process, the electric valve components used in the space propulsion system are all of a threaded structure. Finally, the valve parts are welded and fixed via electron beam welding. The unthreaded structure of electric valve products determines that all processes until welding and fixation require the use of tooling for fixation. The assembly process is more complex and cumbersome than that for general industrial products.
Due to the diverse and small-scale production needs of aerospace valve products, the assembly process has always relied mainly on manual labor. The main process steps in the assembly process of electric valve products are as follows: Parts assembly → valve stroke measurement → electrical performance testing → running-in → beam welding The traditional assembly process has the following problems: (1) In terms of production, efficiency is low and management is cumbersome.
(a) The product needs to be transferred between different workstations or positions, such as assembly, measurement, and testing, resulting in low efficiency. (b) Each set of products requires the entire process of tooling to follow, and during the valve assembly process, the amount of tooling used is large with high costs and cumbersome maintenance. (c) Before electron beam welding of the shell, spot welding and fixation should be carried out through the reserved holes in the fixture. After removing the fixture, circumferential welding should be carried out. One circumferential weld seam needs to be welded in two stages, which is low in efficiency and doubles the welding cost.
(2) In terms of quality, there is a higher dependence on personnel, and measurement errors and assembly consistency are greatly affected by personnel factors, making it difficult to ensure quality consistency.
(a) The disc spring that provides the flipping force and suspension effect during the valve assembly process is compressed by the screw of the fixture. The loading force is indirectly applied through the tightening of the thread. Due to the large discreteness of thread loading, it directly affects the response performance of valve opening and closing, and the influence of this factor cannot be decoupled, requiring repeated disassembly and debugging. (b) As an important indicator parameter, the valve stroke needs to be measured indirectly through a measuring rod using measuring tools. The small pressure exerted by the measuring tool on the adapter rod and the error introduced by manual operation result in low measurement accuracy.
With the rapid development of the aerospace industry, the quantity of aerospace products has significantly increased, and the drawbacks of traditional assembly methods in terms of valve product delivery deadline and quality stability have become prominent. In recent years, intelligent assembly and control systems for products have gradually developed [5,6]. In order to improve production efficiency and yield rate, Zhangchao et al. studied that a multi-way valve automatic assembly system based on industrial robot and Due to the diverse and small-scale production needs of aerospace valve products, the assembly process has always relied mainly on manual labor. The main process steps in the assembly process of electric valve products are as follows: Parts assembly → valve stroke measurement → electrical performance testing → running-in → beam welding The traditional assembly process has the following problems: (1) In terms of production, efficiency is low and management is cumbersome. (a) The product needs to be transferred between different workstations or positions, such as assembly, measurement, and testing, resulting in low efficiency.
Each set of products requires the entire process of tooling to follow, and during the valve assembly process, the amount of tooling used is large with high costs and cumbersome maintenance. (c) Before electron beam welding of the shell, spot welding and fixation should be carried out through the reserved holes in the fixture. After removing the fixture, circumferential welding should be carried out. One circumferential weld seam needs to be welded in two stages, which is low in efficiency and doubles the welding cost.
(2) In terms of quality, there is a higher dependence on personnel, and measurement errors and assembly consistency are greatly affected by personnel factors, making it difficult to ensure quality consistency.
(a) The disc spring that provides the flipping force and suspension effect during the valve assembly process is compressed by the screw of the fixture. The loading force is indirectly applied through the tightening of the thread. Due to the large discreteness of thread loading, it directly affects the response performance of valve opening and closing, and the influence of this factor cannot be decoupled, requiring repeated disassembly and debugging.
As an important indicator parameter, the valve stroke needs to be measured indirectly through a measuring rod using measuring tools. The small pressure exerted by the measuring tool on the adapter rod and the error introduced by manual operation result in low measurement accuracy.
With the rapid development of the aerospace industry, the quantity of aerospace products has significantly increased, and the drawbacks of traditional assembly methods in terms of valve product delivery deadline and quality stability have become prominent. In recent years, intelligent assembly and control systems for products have gradually developed [5,6]. In order to improve production efficiency and yield rate, Zhangchao et al. studied that a multi-way valve automatic assembly system based on industrial robot and machine vision was designed [7]. Wang Zhentao studied an automatic assembly control system of a gas valve to realize accurate positioning of conveyor chain, reliable tightening of threaded parts and overall stable operation of the system of the production line [8]. Zhang Zhihuan developed a valve ball automatic assembly equipment based on LabVIEW platform and NI vision module, which realized the automatic assembly of valve core steel ball and valve body [9]. Yang Mingfang studied the tooling fixtures, and completed the valve body and living door automatic assembly of an inlet check valve of a positive displacement pump [10]; Li Tiecheng studied the assembly machine for reading seats to realize the automatic assembly of valve seats [11]. Zhang Nana, based on the trajectory algorithm, planned the trajectory of the 6-DOF industrial robot, determined the position and posture, and realized the automatic assembly of valve body and valve seat [12]. Wu Wufu developed a single axis automatic tightening equipment to conduct the research on the valve bolt tightening process [13]. In recent years, lot of valve assembly equipment was developed, but most of them can only complete the automatic assembly of a certain process of the valve, and there is little research on fully automated assembly lines [14].
Aiming at the assembly problem of electric valve products in space propulsion systems, a method for online automatic valve assembly involving multiple workstations is proposed. An automatic assembly platform is developed. This article focuses on the hardware development methods and the improvement of key technical capabilities of the automatic assembly platform.

Process Design
In addition to the low efficiency of manual operation, the traditional assembly process of electric valve products also has problems of low loading force accuracy and low measuring accuracy of the spool stroke. These problems led to repeated disassembly and commissioning of products during assembly. Further, the threadless structure of the valve requires fixture clamping during the whole assembly process, which results in a heavy workload in the use and maintenance of the fixture, doubling the EBW workload, and bottlenecking problems during production. To solve these, a preload loading procedure is set up separately in the automatic assembly process to improve the method of force loading and solve the accuracy problem. An improved case connection procedure solves the problem of heavy reliance on tooling and subsequent doubling of the EB welding workload during product assembly. Based on the above analysis, the designed automatic assembly process flow is as follows.
Preloading → valve travel measurement → electrical performance testing → running-in → welding of the shell.

Workstation and Structural Design of the Automatic Assembly Platform
The automatic assembly process flow is divided into five process steps. The mechanical structure is loaded directly using a dynamometer for preloading. The economical and efficient small laser welding method for shell structure connection was selected. Product positioning in space is involved in both the preload loading and shell structure connection processes. Then, these two processes are integrated into one workstation by designing an automatic positioning mechanism. An electrical control system is developed for valve spool stroke measurement and testing, power supply for running-in, and test equipment, and the three procedures are integrated into one workstation. The designed automatic assembly system workstations are a feeding workstation, a second feeding workstation, a loading and welding workstation, and a measuring and testing workstation.
The design of the actuator is based on the station design. The loading and welding stations include a preload loading mechanism, a welding mechanism, and a three-dimensional motion mechanism providing positioning, turnover, and rotation. The measuring and testing station integrates the workstations with the three procedures to form a complete measuring and testing mechanism. A clamping mechanism is designed to meet the requirement of product fixing and positioning for threadless valves in the automatic assembly process. A mechanical arm is used for the automatic operation of the four workstations.

Overall Design
The adopted structure of the automatic assembly production line is a modular design, including an enclosure, display, clean environment control system, electrical control system, and internal work area, as shown in Figure 2. The clean environment control system is integrated on the top of the housing, and its function is to provide positive-pressure filtration for the interior during the operation of the equipment to ensure that the product operation is carried out in a clean environment. The internal work area actuator of the automatic assembly production line is shown in Figure 3, and the design of the internal work area is shown in Figure 4. complete measuring and testing mechanism. A clamping mechanism is designed to meet the requirement of product fixing and positioning for threadless valves in the automatic assembly process. A mechanical arm is used for the automatic operation of the four workstations.

Overall Design
The adopted structure of the automatic assembly production line is a modular design, including an enclosure, display, clean environment control system, electrical control system, and internal work area, as shown in Figure 2. The clean environment control system is integrated on the top of the housing, and its function is to provide positive-pressure filtration for the interior during the operation of the equipment to ensure that the product operation is carried out in a clean environment. The internal work area actuator of the automatic assembly production line is shown in Figure 3, and the design of the internal work area is shown in Figure 4.   complete measuring and testing mechanism. A clamping mechanism is designed to mee the requirement of product fixing and positioning for threadless valves in the automatic assembly process. A mechanical arm is used for the automatic operation of the four work stations.

Overall Design
The adopted structure of the automatic assembly production line is a modular de sign, including an enclosure, display, clean environment control system, electrical contro system, and internal work area, as shown in Figure 2. The clean environment control sys tem is integrated on the top of the housing, and its function is to provide positive-pressure filtration for the interior during the operation of the equipment to ensure that the produc operation is carried out in a clean environment. The internal work area actuator of the automatic assembly production line is shown in Figure 3, and the design of the interna work area is shown in Figure 4.

Automatic Assembly Process Flow
The automatic assembly operation process achieved by the automatic assembly production line is shown in Figure 4.

Design of the Clamping Mechanism
After the assembly of the valve parts, they are fixed inside the clamping structure. The entire process requires their coordination in terms of positioning and transportation between the four workstations (see Figure 4). The clamping structure integrates multiple interfaces, taking into account the functions of valve component assembly fixation, transfer interface, force load pre-tightening, fixation, welding positioning, and electrical connection. Its structure is shown in Figure 5. The function of the clamping claw positioning structure is to provide a connection interface for the clamping of the robotic arm. The function of the positioning pin hole is to mechanically fix it at the workstation. The position sensor is a magnet, which triggers the sensor feedback of each station to install the signal in place after positioning at each station through magnetic force. The compression structure is used to compress and fix the product when it is flipped during the welding process. The power-on connector is a fast plugin electrical connector. When assembling valve parts, the valve wire is connected to the

Automatic Assembly Process Flow
The automatic assembly operation process achieved by the automatic assembly production line is shown in Figure 4.

Design of the Clamping Mechanism
After the assembly of the valve parts, they are fixed inside the clamping structure. The entire process requires their coordination in terms of positioning and transportation between the four workstations (see Figure 4). The clamping structure integrates multiple interfaces, taking into account the functions of valve component assembly fixation, transfer interface, force load pre-tightening, fixation, welding positioning, and electrical connection. Its structure is shown in Figure 5.

Automatic Assembly Process Flow
The automatic assembly operation process achieved by the automatic assembl duction line is shown in Figure 4.

Design of the Clamping Mechanism
After the assembly of the valve parts, they are fixed inside the clamping stru The entire process requires their coordination in terms of positioning and transpor between the four workstations (see Figure 4). The clamping structure integrates mu interfaces, taking into account the functions of valve component assembly fixation, fer interface, force load pre-tightening, fixation, welding positioning, and electrica nection. Its structure is shown in Figure 5. The function of the clamping claw positioning structure is to provide a conn interface for the clamping of the robotic arm. The function of the positioning pin hol mechanically fix it at the workstation. The position sensor is a magnet, which trigge sensor feedback of each station to install the signal in place after positioning at each s through magnetic force. The compression structure is used to compress and fix the uct when it is flipped during the welding process. The power-on connector is a fast in electrical connector. When assembling valve parts, the valve wire is connected The function of the clamping claw positioning structure is to provide a connection interface for the clamping of the robotic arm. The function of the positioning pin hole is to mechanically fix it at the workstation. The position sensor is a magnet, which triggers the sensor feedback of each station to install the signal in place after positioning at each station through magnetic force. The compression structure is used to compress and fix the product when it is flipped during the welding process. The power-on connector is a fast plug-in electrical connector. When assembling valve parts, the valve wire is connected to the electrical connector, and connect it to the conductive interface at the electrical performance testing station. The function of the spring compression structure is to transmit the compression force during the valve product force loading process, compressing the valve core inside the valve. The welding window function is to allow spot welding connection to be performed on the valve shell, so that the subsequent work on the valve product can be separated from fixation. The clamping mechanism has a complex structure, high accuracy, convenience, and durability, and it can adapt to the use of valve products of different sizes within a certain range. This is a necessary condition to achieve automatic manipulation of products across the different workstations.

Design of the Preload Loading, Shell Connection, Motion, and Positioning Mechanism
The pre-tightening force loading and shell structure welding are completed at the same workstation, consisting of a pre-tightening force control mechanism, a welding mechanism, a three-dimensional motion mechanism, a flipping and rotating positioning structure, and a clamping mechanism, as shown in Figure 6. The three-dimensional motion mechanism is composed of a vertical motion mechanism, a horizontal motion mechanism, and a bracket; the flipping and rotating positioning mechanism is designed as a three-degrees-of-freedom structure with a turntable and a horizontal motion platform, as shown in Figure 7. The clamping mechanism ( Figure 6) is fixed on the turntable, which is equipped with positioning, compression, and sensor feedback interfaces to coordinate with the clamping mechanism for positioning. The turntable provides a tightening motion for the pre-tightening force control process, and it provides product circumferential welding positioning and 90 • and 180 • flip positioning for the welding station, achieving welding of both ends and the circumference of the valve. The pre-tightening force control mechanism and welding mechanism are integrated on the three-dimensional motion mechanism bracket, and the combination of flipping and rotating positioning mechanisms can provide planar motion, vertical motion, rotation, and flipping motion with a total of five degrees of freedom for the clamping mechanism of fixed products. This design effectively integrates the pre-tightening force loading with the shell structure welding station.
Appl. Sci. 2023, 13, x FOR PEER REVIEW 6 of 15 electrical connector, and connect it to the conductive interface at the electrical performance testing station. The function of the spring compression structure is to transmit the compression force during the valve product force loading process, compressing the valve core inside the valve. The welding window function is to allow spot welding connection to be performed on the valve shell, so that the subsequent work on the valve product can be separated from fixation. The clamping mechanism has a complex structure, high accuracy, convenience, and durability, and it can adapt to the use of valve products of different sizes within a certain range. This is a necessary condition to achieve automatic manipulation of products across the different workstations.

Design of the Preload Loading, Shell Connection, Motion, and Positioning Mechanism
The pre-tightening force loading and shell structure welding are completed at the same workstation, consisting of a pre-tightening force control mechanism, a welding mechanism, a three-dimensional motion mechanism, a flipping and rotating positioning structure, and a clamping mechanism, as shown in Figure 6. The three-dimensional motion mechanism is composed of a vertical motion mechanism, a horizontal motion mechanism, and a bracket; the flipping and rotating positioning mechanism is designed as a three-degrees-of-freedom structure with a turntable and a horizontal motion platform, as shown in Figure 7. The clamping mechanism ( Figure 6) is fixed on the turntable, which is equipped with positioning, compression, and sensor feedback interfaces to coordinate with the clamping mechanism for positioning. The turntable provides a tightening motion for the pre-tightening force control process, and it provides product circumferential welding positioning and 90° and 180° flip positioning for the welding station, achieving welding of both ends and the circumference of the valve. The pre-tightening force control mechanism and welding mechanism are integrated on the three-dimensional motion mechanism bracket, and the combination of flipping and rotating positioning mechanisms can provide planar motion, vertical motion, rotation, and flipping motion with a total of five degrees of freedom for the clamping mechanism of fixed products. This design effectively integrates the pre-tightening force loading with the shell structure welding station.  The pre-tightening force control mechanism mainly consists of a dynamometer and a tightening structure. The dynamometer is selected from 0-50 N according to the loading range of the valve product with an accuracy of 0.1 N, as shown in Figure 8. The pre-tightening force control mechanism mainly consists of a dynamometer and a tightening structure. The dynamometer is selected from 0-50 N according to the loading range of the valve product with an accuracy of 0.1 N, as shown in Figure 8. The pre-tightening force control mechanism mainly consists of a dynamometer and a tightening structure. The dynamometer is selected from 0-50 N according to the loading range of the valve product with an accuracy of 0.1 N, as shown in Figure 8. The working principle of the pre-tightening force control mechanism is to complete the preload loading through the closed loop control of the vertical displacement of the motion mechanism, the three variables of the dynamometer, and the rotating mechanism. The vertical displacement of the three-dimensional motion mechanism drives the dynamometer to press on the clamping structure of the clamping mechanism ( Figure 6). When the force is reached, the rotation of the turntable of the flipping and rotating positioning structure drives the clamping structure to rotate, tightening the spring compression structure nut of the clamping mechanism to gradually lock the spring compression structure of the clamping mechanism. When the value of the dynamometer changes to 0.1N, the rotation of the turntable stops.
The welding mechanism mainly consists of a laser welding machine, a protective gas supply system, and a visual recognition system. The working mode is a combination of a three-dimensional motion mechanism and a flipping and rotating positioning mechanism to provide planar and vertical positioning, as well as rotation and flipping positioning. Through the visual system, the welding position and focus distance of the weld seam are identified, and welding work is carried out based on parameter design.

Design of the Measurement and Testing Station
The three processes of valve core stroke measurement, electrical performance testing, and running-in are completed at the measurement and testing station. The measurement and testing mechanism consists of an optical measurement device, a motion support, and a positioning support. The process clamping mechanism ( Figure 5) is used to position the valve on the positioning support, as shown in Figure 9. The optical measurement device is mainly composed of high-precision laser sensors, and its minimum test resolution is 0.1 The working principle of the pre-tightening force control mechanism is to complete the preload loading through the closed loop control of the vertical displacement of the motion mechanism, the three variables of the dynamometer, and the rotating mechanism. The vertical displacement of the three-dimensional motion mechanism drives the dynamometer to press on the clamping structure of the clamping mechanism ( Figure 6). When the force is reached, the rotation of the turntable of the flipping and rotating positioning structure drives the clamping structure to rotate, tightening the spring compression structure nut of the clamping mechanism to gradually lock the spring compression structure of the clamping mechanism. When the value of the dynamometer changes to 0.1N, the rotation of the turntable stops.
The welding mechanism mainly consists of a laser welding machine, a protective gas supply system, and a visual recognition system. The working mode is a combination of a three-dimensional motion mechanism and a flipping and rotating positioning mechanism to provide planar and vertical positioning, as well as rotation and flipping positioning. Through the visual system, the welding position and focus distance of the weld seam are identified, and welding work is carried out based on parameter design.

Design of the Measurement and Testing Station
The three processes of valve core stroke measurement, electrical performance testing, and running-in are completed at the measurement and testing station. The measurement and testing mechanism consists of an optical measurement device, a motion support, and a positioning support. The process clamping mechanism ( Figure 5) is used to position the valve on the positioning support, as shown in Figure 9. The optical measurement device is mainly composed of high-precision laser sensors, and its minimum test resolution is 0.1 µm. The function of the motion bracket is mainly to facilitate the installation of optical measurement devices and provide vertical movement, ensuring that the optical measurement device is within the range of the measurement stroke. The positioning support is equipped with positioning, sensor feedback interfaces, and electrical interfaces to cooperate with the position sensor and power on connector of the clamping mechanism ( Figure 5) for positioning feedback and circuit continuity. The workstations of measurement, testing, and running-in processes are integrated. A set of electrical control systems using dynamic acquisition technology is developed. The laser sensor of the optical measurement device detects the position of the internal motion structure of the valve, and the control system provides voltage values at different positions to obtain the voltage and displacement curve of the valve product. ate with the position sensor and power on connector of the clamping mechanism ( Figure  5) for positioning feedback and circuit continuity. The workstations of measurement, testing, and running-in processes are integrated. A set of electrical control systems using dynamic acquisition technology is developed. The laser sensor of the optical measurement device detects the position of the internal motion structure of the valve, and the control system provides voltage values at different positions to obtain the voltage and displacement curve of the valve product.

Overall Application
This equipment is used to conduct process validation on multiple types of electric valves, such as solenoid valves, self-locking valves, and proportional valves, and it was applied to multiple batches of products. Taking the most complex proportional solenoid valve in the assembly process as an example, manual assembly of a batch (10 units) of products takes five days/two people, while using an automatic assembly platform reduces it to two days/person (including product preparation and readjustment time for products not meeting performance standards). A comparison of the running time for each product's manual assembly and the automatic platform's single process is shown in Table 1 (excluding the transfer and scheduling times of the manual assembly process, design and prepare of clamping unit). Table 1. Comparison of single-process operation times between manual assembly and the automatic platform.

Process Steps
Manual Assembly Cycle Automatic Assembly Cycle Efficiency Preload loading 0.5 h 2 min Improved 92% Measurement of valve spool stroke 2 h/2 persons 2 min/1 person Improved 99% Running-in, testing 1 h 2 min Improved 30x Welding of shell structure Electron beam welding twice Electron beam welding once Improved 50%

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,

Overall Application
This equipment is used to conduct process validation on multiple types of electric valves, such as solenoid valves, self-locking valves, and proportional valves, and it was applied to multiple batches of products. Taking the most complex proportional solenoid valve in the assembly process as an example, manual assembly of a batch (10 units) of products takes five days/two people, while using an automatic assembly platform reduces it to two days/person (including product preparation and readjustment time for products not meeting performance standards). A comparison of the running time for each product's manual assembly and the automatic platform's single process is shown in Table 1 (excluding the transfer and scheduling times of the manual assembly process, design and prepare of clamping unit). Table 1. Comparison of single-process operation times between manual assembly and the automatic platform.

Process Steps Manual Assembly Cycle Automatic Assembly Cycle Efficiency
Preload loading 0.5 h 2 min Improved 92% Measurement of valve spool stroke 2 h/2 persons 2 min/1 person Improved 99% Running-in, testing 1 h 2 min Improved 30x Welding of shell structure Electron beam welding twice Electron beam welding once Improved 50%

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 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 ±0.1 N. An example of a force loading curve of a proportional electromagnetic valve is shown in Figure 10.
Appl. Sci. 2023, 13 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.
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 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 ±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.  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 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. 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.

Application of the Shell Structure Welding Process
In the process of manual assembly, each set of products requires a fixture to follow the entire process. Finally, electron beam welding of the shell is carried out, and spot welding and fixation are also required through the reserved holes of the fixture. After removing the fixture, circumferential welding is carried out, thus, the welding cost and workload are doubled. Adding laser welding to the automatic assembly process can not only solve the fixation problem of valves after automatic assembly, but also reduce the high-cost electron beam welding workload by half, alleviate production bottlenecks, and save a large amount of tooling production and maintenance costs. Laser spot welding connection of the shell requires a reasonable design of welding parameters such as the current, voltage, and pulse frequency. The control of the welding point should not only be smaller than the

Application of the Shell Structure Welding Process
In the process of manual assembly, each set of products requires a fixture to follow the entire process. Finally, electron beam welding of the shell is carried out, and spot welding and fixation are also required through the reserved holes of the fixture. After removing the fixture, circumferential welding is carried out, thus, the welding cost and workload are doubled. Adding laser welding to the automatic assembly process can not only solve the fixation problem of valves after automatic assembly, but also reduce the high-cost electron beam welding workload by half, alleviate production bottlenecks, and save a large amount of tooling production and maintenance costs. Laser spot welding connection of the shell requires a reasonable design of welding parameters such as the current, voltage, and pulse frequency. The control of the welding point should not only be smaller than the width of the electron beam-welding seam, without affecting the appearance quality of subsequent electron beam welding, but also be able to compensate for the machining gap error and positioning error of the parts, in addition to ensuring the reliable connection of the valve during transportation and subsequent operation. This is a technical difficulty in this process. Obtained by process verification and the practical application of multiple valve products, an example welding process for connecting the shell of a certain valve product is shown in Figure 15. The welding parameter of No.3 is the optimal.

Quality Improvement of the Valve Core Measurement Process
The switch stroke is one of the most important indicators of a valve. Due to the fact that the movable component of the valve stem is supported by a plate spring cantilever, the valve stem is always in an elastic support state. In traditional assembly methods, manual measurement of the stroke uses a dial gauge to indirectly measure the measuring rod extending from the valve inlet channel. The measuring head of the dial gauge has a certain downward force on the valve stem. When the valve opens, it first needs to overcome this additional force before the valve can open. In addition, due to the inherent gap between the measuring rod and the inlet channel of the valve, the tilt of the valve stem and the movement of the valve core switch can cause inherent deviation in the measurement position of the measuring rod. Figure 16 shows an example of the relationship between the measuring rod, valve core, and valve inlet end of a certain valve with a calculated deviation of about 2µm.

Quality Improvement of the Valve Core Measurement Process
The switch stroke is one of the most important indicators of a valve. Due to the fact that the movable component of the valve stem is supported by a plate spring cantilever, the valve stem is always in an elastic support state. In traditional assembly methods, manual measurement of the stroke uses a dial gauge to indirectly measure the measuring rod extending from the valve inlet channel. The measuring head of the dial gauge has a certain downward force on the valve stem. When the valve opens, it first needs to overcome this additional force before the valve can open. In addition, due to the inherent gap between the measuring rod and the inlet channel of the valve, the tilt of the valve stem and the movement of the valve core switch can cause inherent deviation in the measurement position of the measuring rod. Figure 16 shows an example of the relationship between the measuring rod, valve core, and valve inlet end of a certain valve with a calculated deviation of about 2 µm.
The three valves previously measured manually were measured using an improved non-contact method, and the stroke data showed deviations of −0.020 mm, −0.015 mm, and −0.011 mm, respectively. This has a significant impact on high-precision microstructural valves.
The influence of the above inherent factors, combined with the error-proneness of manual operation, is particularly prominent for micro flow control valves with micrometer stroke and proportional adjustment micro-displacement valves. In addition, for proportional adjustment valves, due to the need to measure values from multiple output points, the manual measurement method requires the cooperation of two people and takes two hours to complete, resulting in low efficiency. By adopting a measurement and testing mechanism, the valve core stroke measurement, electrical performance testing, and running-in process can be continuously completed at the same workstation, avoiding inherent deviations in stroke measurement and improving the measurement accuracy to the visual value of 0.1 µm. Moreover, for proportional valves with a high-testing workload, automatic testing can be completed within 2 min, greatly improving work efficiency. ual measurement of the stroke uses a dial gauge to indirectly measure the measur extending from the valve inlet channel. The measuring head of the dial gauge has a downward force on the valve stem. When the valve opens, it first needs to overco additional force before the valve can open. In addition, due to the inherent gap b the measuring rod and the inlet channel of the valve, the tilt of the valve stem a movement of the valve core switch can cause inherent deviation in the measurem sition of the measuring rod. Figure 16 shows an example of the relationship betw measuring rod, valve core, and valve inlet end of a certain valve with a calculated tion of about 2µm. The three valves previously measured manually were measured using an im non-contact method, and the stroke data showed deviations of −0.020 mm, −0.01 and −0.011 mm, respectively. This has a significant impact on high-precision micr tural valves.
The influence of the above inherent factors, combined with the error-prone manual operation, is particularly prominent for micro flow control valves with mi ter stroke and proportional adjustment micro-displacement valves. In additi  Figure 17 shows the valve displacement curve measured using traditional dial gauge. Figure 18 shows the non-contact measurement curve of valve displacement using an automatically assembled laser displacement sensor. The scale division rate of laser displacement sensor is 0.1 µm. proportional adjustment valves, due to the need to measure values from multiple output points, the manual measurement method requires the cooperation of two people and takes two hours to complete, resulting in low efficiency. By adopting a measurement and testing mechanism, the valve core stroke measurement, electrical performance testing, and running-in process can be continuously completed at the same workstation, avoiding inherent deviations in stroke measurement and improving the measurement accuracy to the visual value of 0.1 µm. Moreover, for proportional valves with a high-testing workload, automatic testing can be completed within 2 min, greatly improving work efficiency. Figure 17 shows the valve displacement curve measured using traditional dial gauge. Figure 18 shows the non-contact measurement curve of valve displacement using an automatically assembled laser displacement sensor. The scale division rate of laser displacement sensor is 0.1 µm.   From the above figure, it is shown that using a laser displacement sensor to automatically measure the valve displacement curve is smoother, the curve has no fluctuations, and the measurement is more accurate. The displacement of the valve using a dial gauge is 161 µm. And the displacement of the valve using laser displacement sensors is 193 µm. The measurement error between the two methods is 32 µm. Therefore, the result by using laser displacement sensors to measure valve displacement is more accurate.  From the above figure, it is shown that using a laser displacement sensor to automatically measure the valve displacement curve is smoother, the curve has no fluctuations, and the measurement is more accurate. The displacement of the valve using a dial gauge is 161 µm. And the displacement of the valve using laser displacement sensors is 193 µm. The measurement error between the two methods is 32 µm. Therefore, the result by using laser displacement sensors to measure valve displacement is more accurate.

Research Summary
(a) By combining the design of a three-axis motion mechanism, a small turntable, and a robotic arm, the product has been transported, positioned, automatically operated throughout the entire process, and the equipment is designed to be miniaturized between multiple workstations. (b) Compared with the original method, through the design of precise control of loading force and non-contact optical measurement method of motion structure, the parameters that affect product performance are precisely controlled and precision is improved, realizing the multivariable decoupling of valve product performance.

Application Value
(a) The automatic assembly production line for electric valve products was verified through application and has a wide range of applications. It can achieve automatic assembly of various electric valve products, including loading force, stroke measurement, running-in, electrical performance testing, and shell structure welding processes. (b) This method can significantly improve assembly efficiency, avoid the need for transfer between multiple departments during manual assembly, and liberate human resources. (c) This method can digitize and manage the process parameters of different products, data and charts output from each process, and improve the level of digital production of products.

Main Innovations
The combination design method of three-axis motion mechanism, small turntable, and robotic arm can effectively solve the problem of product transportation and multi degree of freedom positioning between multiple workstations in the automatic valve production line. This method can be promoted and applied in integrated automatic production lines suitable for workstations, making the design of the production line compact and cost-effective.

Conflicts of Interest:
The authors declare no conflict of interest.