## 5 Test Design and Verification  
The FEM simulation is able to provide guidance for the design of a STACER with the consideration of the friction coefficient, radius of the top rod, and acceleration with an optimized selection. In an experiment, the experimental results can be used to verify the accuracy of the model. After that, the verified model can be used to prove other parameters in the test, such as the friction coefficient. With the development of a STACER in serialization, the simulation model can be reused for a further studies with different parameters. After the simulation analysis of the gathering process of STACER was carried out with the top rod radii of 8.7, 9.0, and 9.3 mm, the force evolution of the top rod during the whole gathering process was shown to be small when the top rod radius was 8.7 mm. During the gathering process, the force value is between 32.4 and 45.1 N, which meets the constant force requirement. In order to verify the reliability and accuracy of the model, a mechanical test on the STACER is conducted to study the gathering process of the spring reel with a top rod radius of 8.7 mm. Figure 14 is an illustration of the test equipment. The STACER consists of a storage bucket, a spring reel, a top rod, and a speed control mechanism. The top rod radius of the STACER is 8.7 mm. Figure 14 is an actual photograph of the STACER. In this experiment, the top rod force is tested during the gathering process. The test diagram is shown in Figure 14. The force sensor of the test system is mounted to the rear of the spring reel by a bracket. The top rod is connected to the speed control system by the cable. The force sensor of the test system is installed on the cable by means of stand base. During the gathering process, the force of the top rod is transmitted to the force sensor through the pulley and the cable so as to collect the force data. The actual test basement is shown in Figure 14. The test basement consists of a zero gravity operating platform, bracket platform, and testing sample (STACER). The STACER is placed on the zero gravity operating platform. A constant velocity motion is required during the gathering process. The SUNDOO SH-2K sensor is applied for the force testing. The measurement accuracy of the sensor is 0.1 N and the sampling frequency is more than 10 Hz. The system software collects sensor data and generates files automatically. In this test, the gathering process repeats four times with the force collection with a gathering speed of 0.042 m/s. The forces of the top rod are shown in Figure 15. During all the four gathering processes, the initial force of the top rod is around 90 N, and the average force of the top rod during the gathering is about 40 N, except in the initial stage with an unstable state. The maximum force in four gathering processes is about 46.3 N and the minimum force is 33.4 N. The overall tendency of the force increases with the gathering length. Figure 13: The morphology of the spring reel under the acceleration condition with X-+100G at (**a**) 0 s; (**b**) 0.24 s; (**c**) 0.72 s; and (**d**) 1 s.  
The experiment results demonstrate that STACER requires a constant force during gathering process with a gathering speed of 0.042 m/s. During the test, the friction force between layers of the spring reel changes under different test conditions. The force on the top rod fluctuates slightly which is less than 15% of the total force. It is similar to the force trend of the finite element result (from 32.4 to 45.1 N) with a top rod radius of 8.7 mm, as shown in Figure 16. The reason for this trend is that the surface friction force is increasing with the gathering length during the gathering process of a STACER. The consistency of the results from the experiment and finite element model verifies the reliability of the finite element model established in this work. This provides a guidance for the further mechanical simulation for the STACER. Figure 14: Equipment illustration of (**a**) STACER; (**b**) test diagram; and (**c**) test basement.