Experimental method research and miniature durability test-bench design of electric sliding door

With recent enhancements in vehicle electrification, electric sliding doors have received significant research attention; however, the testing methods for the sliding doors components need to be improved. Herein, according to the durability evaluation index of sliding doors, a miniaturized durability test bench is designed for the electronic lock system and sliding door power output mechanism assembly. The motion trajectory of the electronic lock system can be fitted as a circular arc curve through dynamic characteristics analysis; therefore, a four-bar linkage with a flexible link is used to simulate the trajectory. Through the finite element simulation method, the test-bench stress is calculated at room temperature and a high temperature (75°C); this facilitates the selection of an appropriate material (GCr15). This experimental method is an economical, reliable, and accurate scheme to standardize the durability test of the electronic lock system and power output mechanism assembly.


Introduction
In the process of electrification and intellectualization, realizing of vehicle's function are transitioning from manual to automatic. The vehicle's door interacts frequently with passengers and play key role for the passengers safety. In recent years, the automation research of vehicle door and its components has gradually deepened.
The common type door adopting now is hinged door which connected with the vehicle body by hinges, there are many extreme working conditions such as overopening and abuse, so the stress condition is random, and the overall service life fluctuates greatly. In addition, the hinged door requires a large opening space, and the obstacle avoidance needs to be considered in the automation design, which makes the design and control more complicated.
Sliding doors are the best choice for fully automatic doors because of their stable movement and small working space requirements. At present, the research of sliding door focuses on the driving mode and its motion analysis. For example, Gu et al. 1 proposed to use temperature-robust air-gap EE-type transformer rails, which can provide power for electric sliding doors. Lu et al. 2 studied the influence of the sliding door opening space on the passenger's entry and exit movement strategy. The above research shows that the sliding door is easy to automate and has a large opening space. The research of electronic locks system cooperates with the automation development of sliding doors. Bai et al. 3 proposed the design of the electronic locks system configuration based on the topology structure. Nottebaum and Bendel 4 proposed an electric inertial locking system to improve the collision safety of door locks. German (Margheritti 5 ) automobile enterprises have also proposed the design of electronic locks system. However, most researches of electronic locks system stays in the design stage, and few experimental methods on the service performance of the door lock are reported.
Among the existing durability test methods, many studies still focus on hinged doors, also body in white (BIW) is usually used to build a durability test-bench. For example, Yuan et al. 6 proposed a durability testbench using ABB robot. Dong and Chen 7 also proposed a mechanical arm-based durability test-bench. These devices take up a lot of space, and specific temperature laboratories are required for high/low temperature durability tests, which leads to obvious cost disadvantages, making many component manufacturers unable to bear. According to Chinese industry standards (QC/T 627-2013 Automobile Electric Door Lock Device 8 and QC/T 1102-2019 Automotive Electric Sliding Door System 9 ), the miniaturized durability testbench device is designed in this paper for the electronic locks system device and the sliding door power output mechanism assembly. By reducing the size of the testbench and reducing the test cost, it can help manufacturers obtain reasonable and effective durability test data, and optimize product design.
The test objects of this test-bench are the electronic locks system and power output mechanism assembly on the electric sliding door. The design requirements of the test-bench are summarized as follows: (1) Electronic locks system: The test-bench needs to simulate the actual working condition of the electronic locks system. Including the sealing force provided by the weather-strip seal and the trajectory fitting of the door opening/closing process. It is required to complete 100,000 opening/closing test cycles of the electronic locks system at room temperature, and 5000 opening/closing test cycles under high/low temperature conditions according to Ministry of Industry and Information Technology of the People's Republic of China (MIIT) standards (QC/T 627-2013 and QC/T 1102-2019). (2) Power output mechanism assembly: At present, the power output mechanism assembly commonly used in electric sliding door is composed of driving motor and transmission chain (it is usually a cable or roller chain), which coordinates with the lower guide rail to drive the door movement. The test-bench shall provide the experiment condition equal to or higher than the actual conditions to ensure the durability test of the driving motor and chain of the door can be carried out reasonably. (3) In addition, the volume of the test-bench is required to be as small as possible. Make it can perform high and low temperature tests by using small or medium-sized high and low temperature experimental devices. At the same time, the design of the test bench is as simple and easy to operate as possible.

Design schedule of the test bench
This test-bench is mainly designed for the durability test of the electronic locks system and power output mechanism assembly of the electric sliding door. Therefore, the sliding door dynamics that affect these two components will therefore be considered first.
(1) The door lock part involves the movement trajectory of the sliding door, the friction force acting on the sliding door, and the sealing force provided by the weather-strip seal; (2) The power output mechanism assembly involves the gravity of the sliding door and the frictional force acting on the sliding door.
The above can be summarized as the dynamic characteristics of the sliding door and the mechanical characteristics of the weather-strip seal.

Motion trajectory fitting of sliding door
According to the actual applying conditions, the wheel arm with one or more wheels in the sliding door moves in the rail located on the vehicle body, as indicated by no. 1, 2, and 3 in Figure 1. The rails can be arranged in the roof, in the side sills or in the side of the body (also on the door). In order to establish a definite static global restraint state, three-connected nonlinear devices should be designed, which can be located on three (Figure 1(a)) or two independent (Figure 1(b)) rails (but they each have only one degree of freedom along the main axis). 10 The rails generate tension H and support force V for the corresponding wheel arm. The upper rail only exerts tension H on the upper wheel arm, thus stabilizing the door movement. It can be seen that the motion trajectory of the sliding door is mainly determined by the middle rail and the lower rail. Therefore, in the fitting of the test-bench, only the motion of the sliding door under the combined action of the middle and lower rails are considered.
According to the above sliding door characteristics and in order to ensure the stability of sliding door movement, the installation planes of the middle and lower rail need to be parallel to each other. The sliding door's movement diagram is shown in Figure 2, the sliding door 4 is hinged with the wheel arm 3, and has a rotation freedom in one direction. The wheel arm 3 cooperates with the rails 1 and 2 through the wheels, and moves along the rail direction. Figure 2(a) to (c) show the closed state of the door, the opening process of the door, and the opened state of the door, respectively.
Referring to components in the sliding door of a model Multi-Purpose Vehicle (MPV) and combining with Figure 2, the actual components can be simplified to replace the corresponding simplified mechanism on the right side of Table 1. For describing the motion process of the sliding door, the simplified model can reduce the difficulty of simulation. Through this simplified model, the movement trajectory of any point on the electric sliding door can be obtained (the amount of deformation during the movement of the sliding door is much smaller than its overall size, so its deformation can be ignored).
The multi-body dynamics simulation of the simplified model is carried out by Adams software, and the trajectory of the lock position during the door movement is obtained. The working stage of the electric door lock starts from the contact between the door lock and the lock bolt, and ends when the door is completely closed. By fitting the movement trajectory of the electric door lock position at this stage, the arc curve of the electric door lock during the working process can be obtained, as shown in Figure 3. The arc curve fitting formula is as formula (1) and (2), and the fitting arc curve radius is obtained by formula (3) x According to the above formulas, the arc curve of the working process of the MPV's electric door lock is obtained, and the radius is r F = 155:81mm. The trajectory fitting results and correlation are shown in Table 2. According to the relevancy of each data, the highprecision fitting of the working trajectory of the electric door lock can be realized.
In order to simulate the working trajectory of the electric door lock, the motion mechanism of the test bench is designed as a parallel four-bar mechanism, in which the flexible connecting rod ensures that the length of the rod can be adjusted according to the actual fitting curve. The working trajectory radius of the electric door lock in the test bench is the rod length, that is, r F = L R , and the length of the connecting rod of the test bench is adjusted to L R = 155:81mm. The electric door lock is assembled at an angle to the electric sliding door, so the electric door lock mounting plate in the test bench will be configured at the same angle, as shown in Figure 4. By adjusting the length of the connecting rod L R , the purpose of simulating the movement trajectory of different models electric door lock in the working process is achieved. And all sliding door models can be fitted by this method.

Motion resistance simulation of sliding door
During the movement of the electric sliding door, the resistance it receives mainly coming from the friction between the wheel arm roller and the guide rail. To obtain the resistance data, the tension meter has been used to measure under horizontal and slope movement conditions. The average pulling force F F required for move at a constant speed under horizontal movement is 59.2 N; the average pulling force F S required for move at the same constant speed under 10% slope is 97:44 N . The relationship of the experimental data is described as formula (4), thus the door mass m D and the comprehensive friction coefficient m between the wheel arm roller and the rail can be calculated After calculating According to the obtained door mass m D = 39:57 kg and comprehensive friction coefficient m = 0:15, the friction disks is adopted on the test bench to provide movement resistance, shown in Figure 5.
According to the movement distance of the electric sliding door, the length of the roller chain in the test bench is designed. By measuring from closing to fully opening, the total movement distance of the sliding door L M is 119 7mm. This roller chain is in contact with the lower rail and is subjected to the pulling force from the sliding door motor and the support force from the lower rail. It is the most severely stressed component in the power output mechanism assembly. Therefore, we will mainly evaluate the durability of this chain link, as shown in Figure 6: The roller chain used in the sliding door system has each chain link length of l C = 9:5mm as shown in Figure 7. So, the number of roller chains are used in the test bench is L M =l C = 126, cooperating with the motor and friction disk to form the power output mechanism assembly of the test bench.
The working process can be described as follows: (1) The chain drive motor cooperates with the driving sprocket on the motor mount to provide driving force for the chain. Test and calculation of sealing force Automotive door system weather-strip seals play a major role in determining door closing effort, isolating the passenger compartment from water and reducing the wind noise inside the vehicle. 11 And the weatherstrip seals are typically extrusion bulbs made of elastomers that are attached to either the car door or the car body in order to seal the passenger compartment. The sealing strip runs usually all around the perimeter of the car door. 12 The sealing strip provide sealing force when the door is closed, and the door needs to overcome this resistance during the closing process. The sealing strip generally has two parts: core and coreless. The sealing strip will produce an air cushion phenomenon and increase the reaction force. The sealing strip is designed with holes regularly arranged and distributed linearly along the sealing strip to speed up the air flow in the cavity when the door is closed. However, , the smaller the value, the higher the fitting relevancy; Reduced Chi-sqr: residual sum of squares, the smaller the value, the higher the fitting relevancy; R: relevancy.
when the sealing strip is compressed, the gas in the cavity overflows to generate a nonlinear damping force. 13 Manual closing of the doors often requires a faster initial velocity, which results in resistance to the air compression in the body. 14 The electric sliding door is driven by a motor, so the speed is controlled stably, it is less affected by the air pressure. Therefore, in the process of closing the sliding door, only the force of the sealing strip on the sliding door is considered.
In the test, five sections of coreless and cored samples with l s length were respectively measured, and the opening of the sealing strip was effectively sealed to obtain the effect of the damping force when the air overflowed. The Compression Load Deflection (CLD) curve of the sample measured by the universal test machine is shown in Figure 8. (When we measure the CLD curve of the sealing strip, we set the speed of universal mechanical testing machine according to the actual working situation. Under the experimental conditions, the CLD curve of the sealing strip is extracted, which is consistent with the actual work.) According to measure in the real car, when the sliding door is closed, the average compression of the sealing strip is x c = 5:78mm. It can be seen from the CLD curve that when the compression of the sealing strip is 5.78 mm, F Coreless = 7:4N , F Cored = 42N . The sealing force of the entire sealing strip can be calculated, and the specific settlement process is shown in formula (6): where: l Coreless (mm) is the length of the coreless sealing strip; l Cored (mm) is the length of the cored sealing strip; F Sealing Force (N) is sealing force provided for sealing strip; f x (N) is the elastic reaction force of the sealing strip per unit length when the compression amount is x.
The spring resistance system designed in the test bench consists of springs distributed at four corners, and the springs are connected with the test bench by bolts. In the process of sealing force simulation, the compression amount of the spring can be changed by adjusting the position of the bolt, so as to simulate the sealing force effect of different models. The total elastic force of the four springs after they are fully compressed should be greater than or equal to the sealing force provided by the sealing strip (493.6 N), so the precompression force of the four springs is set to 125 N. The spring resistance system of the test bench is designed according to the calculated sealing force, and the selected spring model is NT-SWS26-70, with 26 mm

Overall design of durability test bench
Overall design of the durability test bench is completed as shown in Figure 9. The test bench consists of four parts: bench system, powertrain system, spring resistance system, and door system.
(1) The bench system plays a major bearing role.
The rail, power output mechanism assembly, spring resistance system, and door system are installed on the bench system.
(2) Power assembly system provides power for test bench. This system installs the sliding door power output mechanism assembly used in the actual electric sliding door on the test bench in order to testing the durability of the power output mechanism assembly. (3) The spring resistance system in the test bench can change the force of the spring resistance system by adjusting the spring compression to achieve the effect of the sealing force on the door. (4) The door system consists of flexible connecting rods, an electric door lock mounting plate, and a counterweight that simulates the door mass. The fitting of sliding doors of different vehicle types can be achieved by adjusting the length of flexible connecting rods, the angle of mounting plate of electric door lock, and the mass of counterweight.
The overall size of the test bench is 600 mm 3 540 mm 3 580 mm, meeting the size requirements of small temperature boxes. The adjustable parts in the test bench make the test stand universal and reach the goal of standardization of test methods.

Material selection of test bench based on finite element analysis
After the test bench design completed, finite element simulation is carried out. Through the simulation results, the stress of the test bench is evaluated and the correct materials are selected to ensure that the strength can meet multiple endurance test cycles.  (1) The 3D model of the bench system and door system of the test bench is exported from Solidworks software. (2) Importing 3-D model files of test bench into Hypermesh software for structured meshing. (3) The structured meshing files are imported into Abaqus software and loaded at room and high temperature according to the actual working conditions of the test bench for static simulation. (4) Obtaining the static stress simulation result, analyzing the stress cloud diagram, thus selecting appropriate material. Figure 10(a) shows the structured meshing of the test bench in Hypermesh software. The bench system uses surface meshes and the whole system mainly uses tetrahedral meshes. The door system is divided into solid meshes, all using hexahedral meshes. The bench as a whole adopts the material constitutive of ball-bearing steel. And assign the gantry system to the shell element section and the door system to the solid element section, as shown in Table 3.
Then according to the structure, assign the corresponding kinematic pair, and assign boundary conditions and loads. Figure 10(b) is the setup in Abaqus software, the degrees of freedom of the four bolt holes of the base and the bottom surface are completely constrained; apply 125 N load (i.e., the elastic force provided by the four springs simulating the sealing force) to the bolt holes at the installation position of the spring resistance system and the corresponding positions on the door system where the spring force is applied (four places on the bench system and four places on the door system), and add gravity to the door system. Bond the installation hole of the electric door lock position in the door system to the lock bolt in the bench system. After applying loads and boundary conditions, temperature fields were added to the model to obtain the stress distribution of the test bench at room temperature (20°C) and high temperature (75°C).
The pre-processing is completed, submit the solution to obtain the stress cloud diagram as shown in Figure 11. The simulation results show that at room temperature, the stress distribution of the test bench except for the bolt parts is below 52.46 MPa (Figure 11(a)), and the overall stress is within a safe range. However, the maximum stress value of the bolt parts can reach 265.5 MPa, as shown in Figure 11(b).
In the high temperature environment, the base, bearing seat, electric door lock mounting plate ( Figure  11(c)), and lock bolt parts (Figure 11(d)) of the test bench system have higher stress. The maximum stress is 342.9 MPa (appearing at the base weld position), and the overall stress distribution is mainly between 114 and 200 MPa. The main reason for the increase in stress of the gantry system is the thermal stress caused by the temperature increase. Compared with the bolt parts, the structure of the bench system is more compact, and there are boundary conditions to limit its motion. Therefore, in the process of thermal expansion, the mutual extrusion of the structural parts is more obvious, which leads to the increase of the stress. According to the simulation analysis results, the GCr15 (with S ł 0.020%; P ł 0.027%; Cr: 1.40%-1.65%) material selected for the bench system is finally determined.

Durability test evaluation of the test bench
In the test, the test bench supplies power to the powertrain system motor and the electric door lock motor through a 12 V constant voltage power. One running cycle of the test bench is 12 s, and is counted by the photoelectric sensor. Figure 12(a) shows the 62,088th test under the normal state of an electric door lock. Figure 12(b) shows the signal monitoring platform and the BIW test bench for the real vehicle test. The test can detect the experimental situation through the current signal drawn from the power supply and the camera located next to the test bench. The feasibility of the  test bench can be further verified by comparing the current curves of the two groups of experiments. Figure  13(a) shows the current of the powertrain system motor in two groups of experiments. Figure 13(b) shows the current of the electric door lock motor in two groups of experiments. The experimental curve has a highly consistent trend. The test bench has now passed dozens of normal, high, and low temperature and constant temperature and humidity durability tests for durability test, and it runs well and stably.

Conclusion
The paper aims at the durability test requirements of the electric door lock and the power output mechanism assembly. The dynamic characteristics of the automobile sliding door and the mechanical characteristics of the sealing strip are analyzed, and the experimental method with high reliability is proposed. The design of a small durability test bench for key components of the sliding door has been completed: (1) Using the simplified sliding door model, the trajectory fitting of the door lock position of the sliding door is completed. Its trajectory can be accurately fitted into a circular arc curve, which determines the test bench structure with a parallel four-bar mechanism as the main body. (2) Through experiments and theoretical calculations, the mass of the sliding door body and the comprehensive friction coefficient between the roller of the wheel arm and the guide rail can be obtained. The powertrain system design and door system weight calibration are completed. (3) Through experiments and calculations, the CLD curve of the door sealing strip was measured, and the value of the sealing force provided by the sealing strip was calculated. The design of the resistance system of the test bench is completed, and the spring type in the resistance system is selected. (4) Finally, through finite element simulation, the strength of the bench system of the test bench is analyzed, and the material is selected as GCr15 bearing steel.
Through the above method, the paper miniaturizes the BIW test bench commonly used in the durability test of electric door locks into a small test bench that meets the size of a small and medium-sized temperature box. The high and low temperature durability test of the electronic locks system and the sliding door power output mechanism assembly can be carried out in a small and medium-sized high and low temperature test box. Greatly reduces the high and low temperature test cost of the electric door lock and power drive mechanism assembly. In the test, the test bench well achieved the durability test purpose of the electronic locks system and sliding door power output mechanism assembly, thus providing a fast, economical, and effective durability test method for the manufacturer. Through this experimental method, a series of test parameters are obtained, which can quickly detect the problems of existing products, which can play a guiding role in the design and development of new door locks.