## 2 Materials and Methods
### Determination of Contact Parameters
#### 2.2.1 Determination of the Collision Restitution Coefficient  
A Q235 steel plate and oil sunflower 50-hole seedling pot were selected for the test. The free-falling drop test was carried out on the plug seedling pot, as shown in Figure 3 below.  
Figure 2: Variation of morphological characteristics of oil sunflower of 50-hole oil sunflower plug seedling (**a**) Plant height; (**b**) Leaf spread; (**c**) Stem diameter; (**d**) Single plant mass.  
The middle part of the pot of the oil sunflower 50-hole plug seedling was marked with pigmented dots to facilitate subsequent capture of the movement trajectory. The stalks of the seedlings were clamped with a gripper and released from a height of 350 mm to free fall and collide with a steel plate (or potting substrate block) located directly below the base. A Pco.dimax S4 high-speed camera was placed at a suitable distance and height in front of the iron stand to capture the whole process. After the test was completed, the motion of the pot during each collision was tracked by TEME software to determine its trajectory characteristics. In this process, to facilitate the relationship image processing of displacement using TEME, it is necessary to first establish a two-dimensional coordinate system on the test bench and measure the distance from the high-speed camera to the coordinate system and the seedling pot. The distance between the high-speed camera lens and the test surface is 2000 mm, the horizontal distance is set to 400 mm, and the vertical distance is set to 700 mm in TEME. The distance between the high-speed camera lens and the test surface is 2000 mm.  
TEMA software was used to analyze the motion process of the filmed pot, plot the obtained displacement-time curves of the pot-pot and the pot-steel plate collision motion, and then determine the collision restitution coefficient, as shown in Figure 4.  
Figure 4: Displacement–time curve of the collision between pot and measured object.  
Figure 3: Principle schematic of restitution coefficient measuring device.  
The collision recovery factor is defined as the ratio of the relative velocities to the contact points before and after the collision [27].  
Before the collision, the seedling pot made a free fall motion, ignoring the influence of air resistance, according to the kinematics formula:  
\[v_{\text{on}}=gt_{1} \tag{1}\]  
\[h_{1}=\frac{1}{2}gt_{1}{}^{2} \tag{2}\]  
where \(v_{\text{on}}\) is the instantaneous velocity of the seedling pot before the collision, m/s; \(g\) is the acceleration due to gravity, m/s; \(t_{1}\) is the time of the first free fall, s; and \(h_{1}\) is the height of the free fall, mm.  
After the collision, when the seedling pot reaches the highest point, the speed will drop to zero according to the motion formula:  
\[v=v_{0}+gt^{2} \tag{3}\]  
\[h2=v_{0}t_{2}+\frac{1}{2}gt_{2}{}^{2} \tag{4}\]  
where \(v\) is the speed at which the rebound reaches the highest point after the collision, m/s; \(v_{0}\) is the instantaneous velocity of the oil sunflower seedling pot after the collision, m/s; \(t_{2}\) is the time when the rebound reaches the highest point after the collision, s; and \(h_{2}\) is the height of the highest point after the collision, mm.  
From the definition of the collision restitution coefficient, the collision restitution coefficient is  
\[\text{e}=\frac{v_{0}}{v_{\text{on}}}=\sqrt{\frac{h_{2}}{h_{1}}} \tag{5}\]  
After several tests, the range of the collision restitution coefficient between the pot and the pot is 0.29-0.36, and the range of the collision restitution coefficient between the pot and steel is 0.18-0.27.