## 1 Introduction  
Compared with open-field direct seeding and mulching sowing, seedling transplanting has obvious advantages in terms of economic crop planting in Inner Mongolia [1; 2; 3]. Oil sunflower is a typical crop with a short growth cycle, wide adaptability, and drought resistance. It has become the fourth-largest oil crop after soybean, peanut and rape in China [4; 5; 6]. During mechanized transplanting, the plug seedlings need to go through the processes of picking up, feeding, carrying, and planting. The plug seedling will inevitably collide, impact, and be squeezed with the key component of the transplanter. The pot will be thus damaged, which will seriously affect the transplant survival rate. Therefore, it is urgent to study the mechanical properties of drop impact of plug seedlings. Many studies have mainly focused on matrix loss and collision movements between plug seedlings and planners. [PERSON] et al. [7] designed a multipin flexible seedling pick-up gripper and conducted transplanting tests, obtaining 93.37% integrity of the plug seedling substrate. [PERSON] et al. [8] studied the effect of the clamping speed of the seedling gripper on the success rate of seedling picking and optimized the clamping parameters to improve the success rate of seedling picking and reduced the damage to the pots. [PERSON] et al. [9]designed a robotic arm that can grab and drop 20 seedlings per minute. The seedling-picking system developed based on the robotic arm has a simple structure and effectively reduces matrix loss. [PERSON] et al. [10] studied the effects of the matrix ratio, water content, and pot quality on seedling extraction and the matrix breakage success rate. [PERSON] et al. [11] studied the collision motion between the plug seedling and the planter, established the different equations of the motion of the plug seedling at each stage, and verified the reliability of the established model through high-speed photography experiments. [PERSON] et al. [12] performed a high-speed photographic analysis of the collision movement process of the planting device and the seedlings falling to the bottom of the planter established a dynamic model at each stage. [PERSON] et al. [13], utilizing [PERSON]'s theory, obtained the maximum contact force equation of the seedling pot and the impulse and impulse moment theorems to obtain the equation of motion of the collision between the pot and seedling stem and the planting device. The contact stress distribution of hole tray seedlings under load is directly related to the pot damage of hole tray seedlings during transplanting. At present, there are three main methods to study agricultural material damage: (1) Velocity variation laws measured by velocity (acceleration) sensors to determine the pot damage during material collisions [14; 15]. (2) Using a pressure-sensitive film to analyze the damage in the process of collision [16; 17]. (3) Combined with analysis software, the film pressure sensor is used to analyze the collision process of the material [18; 19]. Compared with the first and third schemes, the second scheme uses a flexible film network tactile pressure sensor and an I-Scan System data processing system, which can visually analyze the contact stress distribution of the plug seedlings during the collision process and explore the plug seedling impact damage mechanism. This paper used T562 oil sunflower plug seedlings as the research object. The drop impact test of the plug seedlings was carried out with the matrix loss rate as the test index. To further study the collision impact force on the pot damage, the Tekscan pressure distribution measurement system was used to measure the collision impact force, and the orthogonal tests were conducted under different factors to obtain the influence degree of each factor on the collision impact force. The significant influencing parameters were deeply analyzed to obtain the regression equation between the contact stress distribution and the pot damage.