Experimental Study on Pot Damage and Contact Stress Distribution Characteristics of Oil Sunflower Plug Seedlings

Abstract

To reveal the collision damage mechanisms of plug seedlings and improve the quality of seedlings, the kinetics equations of the plug seedlings were established based on the generalized Hertz-theory. The influence laws of different factors on pot damage were obtained through a drop impact test. The Tekscan pressure distribution measurement system measured the collision impact force, and the orthogonal tests were conducted. The test showed that the influence of the collision impact force was on the order of plug specification > drop height > contact material. The Tekscan pressure distribution measurement system measured the change law of contact stress distribution under significant influencing factors. The test results showed that the collision contact area between the plug seedlings and contact materials from large to small was soil, steel, and ABS plastic. The collision contact area between the plug seedlings and other plug specifications was 50 plug, 72 plug, and 105 plug from the largest to the smallest. When the plug seedlings collided with contact materials, the average contact stress between the seedlings and the steel plate ranged from 19.4 kPa to 22.8 kPa. When the plug seedlings of various sizes collided with steel plates, the average contact stress was ordered as 105 plug, 72 plug and 50 plug in descending order. A linear regression model between collision impact force and matrix loss rate under different factors was established based on the pressure data collected by the Tekscan pressure distribution measurement system. This study provides a basis for exploring the impact damage mechanisms of plug seedlings and improving the seedling quality.

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 planters. Han et al. [7] designed a multipin flexible seedling pick-up gripper and conducted transplanting tests, obtaining 93.37% integrity of the plug seedling substrate. Jin 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. Paradkar 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. Jiang et al. [10] studied the effects of the matrix ratio, water content, and pot quality on seedling extraction and the matrix breakage success rate. Chen 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. Jin 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. Liu et al. [13], utilizing Hertz’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. This study provides a basis for exploring the impact damage mechanisms of plug seedlings and improving the seedling quality.

2. Materials and Methods

2.1. Test Material

In this test, T562 oil sunflower plug seedlings were grown by the Inner Mongolia Heyuan Agricultural Technology Company. The seedling matrix consisted of grass carbon, vermiculite, and pearlstone with a mass ratio of 3:1:1 [20]. Different specifications of oil sunflower plug seedlings were cultivated, and the seedling age was 30 days. The moisture content was measured by the drying method at 58.78–62.47%. The oil sunflower plug seedlings grew normally, with well-developed root systems knotted together with the seedling matrix to form a strong and flexible pot, as shown in Figure 1.

2.2. Collision Impact Force Analysis of Oil Sunflower Plug Seedlings

2.2.1. Analysis of Collision Impact Force Change Process of Oil Sunflower Seedlings

The plug seedling is a typical nonlinear viscoelastic material. This paper is based on the analysis of the generalized Hertz-theory in the ideal state [21,22]; there is a damping effect in the collision contact stage, and the maximum compression lags behind the maximum collision impact force. As shown in Figure 2, when the seedling fell from a certain height and collided with the contact material, the collision impact force gradually increased from zero. The pot is in a compression state. When the collision impact force reached the maximum value, it started to unload and slowly reached 0.

2.2.2. Theoretical Analysis on Contact Force of Oil Sunflower Plug Seedlings

It was assumed that the physical parameters of plug seedlings were consistent, ignoring the influence of air resistance and other factors. The plug seedlings were only affected by gravity during the falling process. The kinetic energy theorem obtained the normal velocity of the seedlings before the collision. According to the kinetic energy theorem:

$$\mathit{mgh}=\frac{1}{2}{\mathit{mv}}_1^2$$

(1)

where m is the mass of oil sunflower plug seedlings, g; g is the acceleration of gravity, m/s²; h is the drop height, m; v₁ is the initial collision velocity, m/s.

Including:

$${\mathit{v}}_1=\sqrt{2\mathit{gh}}$$

(2)

According to the impulse theorem:

$${{\displaystyle \int }}_{t_1}^{t_2}F\mathrm{dt}=m\Delta v$$

(3)

where: t₁ is the starting time of the collision, s; t₂ is the collision contact time, s; F is collision impact force, N; ∆v is the change of velocity before and after the collision, m/s.

Therefore,

$${{\displaystyle \int }}_{{\mathit{t}}_1}^{{\mathit{t}}_2}{\mathit{F}\mathrm{dt}=\mathit{m}(\mathit{v}}_2-{ \mathit{v}}_1)$$

(4)

where: v₂ is the collision velocity of the plug seedling in t₂ time, m/s

When v₂ is equal to 0, the compression amount of the pot reaches the maximum, and the corresponding time is t_s.

$${{\displaystyle \int }}_{{\mathit{t}}_1}^{{\mathit{t}}_{\mathrm{s}}}{\mathit{F}\mathrm{dt}=\mathit{mv}}_1=\mathit{m}\sqrt{2\mathit{gh}}$$

(5)

It can be seen from Figure 2 that the area of the curved edge formed by the collision impact force F and the time t₁~t_s is approximately equal to the rectangular area of the side length t_s−t₁ and F_m.

$${{\displaystyle \int }}_{{\mathit{t}}_1}^{{\mathit{t}}_{\mathrm{s}}}{\mathit{F}\mathrm{dt}=\mathit{k}}_0{\mathit{F}}_{\mathrm{m}}{(\mathit{t}}_{\mathrm{s}}-{\mathit{t}}_1)$$

(6)

where k₀ is the area coefficient.

From Equations (5) and (6), it follows that:

$${\mathit{F}}_{\mathrm{m}}=\frac{\sqrt{2\mathrm{g}}}{{\mathit{k}}_0\left({\mathit{t}}_{\mathrm{s}}{-\mathit{t}}_1\right)}\mathit{m}\sqrt{\mathit{h}}$$

(7)

In Equation (7), k₀, t₁, and t_s are all related to the test and contact materials. Therefore, the maximum collision impact force F_m is mainly associated with the quality of the plug seedlings, drop height, contact material, and other factors.

2.3. Drop Impact Test on Oil Sunflower Plug Seedlings

As shown in Figure 3, the drop impact test between the plug seedlings and the steel plate was mainly composed of a seedling gripper, frame, handle, 5250 flexible thin-film-network tactile pressure sensor, and computer. The pressure distribution measurement system was composed of a 5250 flexible-film-network tactile pressure sensor, a handle, and an I-Scan System [23]. A flexible-film-network tactile pressure sensor was arranged in the test plane. A calibration file needed to be imported and zeroed before each test began. The sensor surface was cleaned of the residual loss matrix after each drop impact test.

This test mainly adopted the working principle of free fall, which was divided into three cases. The plug seedlings collided with the soil. The soil thickness in the test soil bin was 16 cm, and the width was 37 cm. The soil was sandy loam, the moisture content was 14.52%, and the soil compaction was 90~100 N/cm². To protect the sensor and ensure consistent buffering properties of the soil, the soil surface was wrapped with plastic film, the sensor was placed on the surface of the film, and a thin layer of soil was laid flat on top of the sensor, as shown in Figure 3a; for the collision between the plug seedling and the steel plate, a steel plate was placed on the test plane, and sensors were arranged on the steel plate, as shown in Figure 3b; the plug seedling collided with the ABS plastic, an ABS plastic plate was placed on the test plane, and the sensor was arranged on it. The steel plate in Figure 3b was replaced with an ABS plastic plate. The properties of the three contact materials are shown in

Table 1. Properties of three contact materials.

Type of Material	Thickness/mm	Density/(g/cm3)	Elastic Modulus/GPa	Poisson’s Ratio
Steel	5	7.85	182	0.3
ABS plastic	5	1.07	2.2	0.39
Soil	160	1.452	0.00284	0.42

[24]. To make the oil sunflower seedlings drop vertically as much as possible, the seedling gripper was used to clamp the stem at the junction of the plug seedlings and the pot. The drop height (h) is the distance from the bottom of the pot to the test plane, as shown in Figure 4.

2.4. Testing Principle and Method of the Flexible Film Network Tactile Pressure Sensor

As shown in Figure 5, the sensor size was 245.9 × 245.9 mm, the spatial resolution was 3.2/cm², the pressure measurement range was 0~0.179 MPa, and the scanning frequency was 0~100 Hz. The flexible thin-film-network tactile pressure sensor was a matrix-based thin film pressure sensor consisting of 2 very thin polyester films. Many rows of strip conductors were laid on the inner surface of one film, and some columns of strip conductors were laid on the inner surface of another film. Many horizontal and longitudinal conductors were crossed to an array of stress-sensing points [25,26].

After the oil sunflower plug seedlings collided with different contact materials, the flexible film-network tactile pressure sensor scanning circuit scanned each sensing point. The resistance value of each force sensing point was measured by the I-Scan System data processing system and converted into data such as contact stress, contact area and contact stress peak value.

2.5. Pot Damage Measurement Method

To explore the law of pot damage, it was necessary to study the matrix loss of the pot before and after the drop impact of the plug seedlings. When the matrix loss of the plug seedlings was greater, the damage to the pot was greater. In contrast, when the matrix loss of the plug seedlings was less, the damage to the pot was smaller. The matrix loss rate (K) represents the percentage of the mass of the seedlings after the drop and the mass of the matrix before the drop [27], as shown in Formula (8):

$$\mathit{K}=\frac{\mathit{M}-{\mathit{M}}_1}{\mathit{M}} \times 100\%$$

(8)

where K is the matrix loss rate, %; M is the mass of plug seedlings before the drop impact, g; and M₁ is the mass of plug seedlings after the drop impact, g.

2.6. Orthogonal Test and Significance Analysis

Taking the matrix loss rate as the test index, the impact damage test of the plug seedlings was carried out under different factors. To further study the collision impact force on the pot damage, the collision impact force was used as the test index, and different plug specifications, contact materials, and drop heights were used as the test factors. The Box-Behnken module of Design-Expert 8.0.6 was used to design an orthogonal test with 3 factors and 3 levels. Seventeen groups of tests were carried out. The test factors and levels are shown in

Table 2. Test factors and levels.

Factors	The Level of Coding
	−1	0	+1
Plug specification	50 plug	72 plug	105 plug
Contact material	Steel	Soil	ABS Plastic
Dropping height/mm	50	150	250

. Each group of drop impact tests was repeated 20 times, and the average value was taken as the final value.

As the sensor is a matrix-based membrane pressure sensor, many transverse and longitudinal conductors cross to form a stress-sensing point array. Therefore, the contact stress in this paper is the average value of the stress at many impact contact sensing points. The average contact stress (P) and contact area (A) of the oilseed sunflower plug seedlings had a significant effect on the pot damage. The product of the average value of the contact stress and the contact area is the collision impact force [28], as shown in Equation (9):

$${\mathit{F}=\mathit{P} \times \mathit{A} \times 10}^{-3}$$

(9)

where F is the collision impact force, N; A is the contact area, mm²; and P is the average contact stress s, kPa.

The influence and significance of each factor on the impact force were obtained through the analysis of the orthogonal test results, and the significant influencing parameters were analyzed to obtain the regression equation and determination coefficient between the contact stress distribution and the pot damage.

3. Results and Discussion

3.1. Variation Law of Collision Impact Damage of the Pot

3.1.1. Variation Law of the Collision Impact Damage under Different Contact Materials

The seedling age was 30 days, and 72 plug seedlings with the same overall growth were selected to carry out the drop impact test between the oilseed sunflower seedlings and steel, soil, and ABS plastic at different heights. The test results are shown in Figure 6. The matrix loss of the seedlings increased linearly with increasing drop height. When the drop height was less than 150 mm, the matrix loss of oil sunflower seedlings on the steel plate, soil, and ABS plastic was small. When the drop height was over 150 mm, the loss of the seedling matrix of the oilseed sunflower hole tray was obvious. The loss of the steel plate was larger, followed by the soil, and the plastic plate was the smallest. The main reason was that the plastic deformation of the steel plate was smaller when the seedlings collided with the steel plate, making the pot more deformed, and the matrix loss when the seedlings collided with the steel plate was larger than other materials. Although the soil had a certain hardness, there were still many pores inside. The contact area was large during collision, which reduced the deformation; therefore, the matrix loss between the seedling and the soil was at an intermediate level. When the oil sunflower plug seedlings collided with the ABS plastic, the plastic had a certain elasticity, which buffered the collision and deformation of the seedlings. The plug seedlings had less pot damage and less matrix loss. The damage to the pot of the seedlings caused by different contact materials was related to the contact stress, the contact area, and the collision impact force.

3.1.2. Variation Law of Collision Impact Damage under Different Plug Specifications

The seedling age was 30 days, contact material being a steel plate, and the overall growth pattern were selected, and the drop impact test of oil sunflower plug seedlings under different plug specifications was carried out. The test results are shown in Figure 7. The matrix loss of seedlings increased linearly with increasing drop height. Among them, the 50 plug seedlings had the largest matrix loss, the 72 plug seedlings were the second, and the 105 plug seedlings had the least substrate loss. The main reason for this was that with the increase in the drop height, the contact stress and contact area mechanisms under different plug specifications of the plug seedlings and the steel plate were different, which eventually aggravated the pot damage, and the matrix loss gradually increased.

3.2. Establishment of Collision Impact Force Model and Regression Analysis

The drop impact test of the seedlings was carried out with different test factors (contact material, drop height and plug specification). As shown in

Table 3. Box-Behnken test design and results.

No.	Test Factor Level Value			Collision Impact Force/N
	X1	X2	X3
1	0	0	0	6.08
2	−1	−1	0	7.42
3	0	1	1	5.77
4	0	0	0	6.08
5	1	1	0	4.62
6	1	0	1	6.55
7	0	0	0	6.18
8	0	−1	1	6.39
9	−1	0	1	8.66
10	0	1	−1	4.48
11	−1	1	0	6.81
12	1	−1	0	5.08
13	−1	0	−1	7.33
14	0	0	0	6.08
15	0	0	0	6.58
16	0	−1	−1	4.63
17	1	0	−1	4.33

, orthogonal tests were designed by Box-Behnken. Multiple regression fitting analysis was performed on the test results. A quadratic multinomial regression model of the plug specification (X₁), contact material (X₂), and drop height (X₃) was established, as shown in Equation (10).

$${\left\{\begin{array}{c}\mathit{F}=6.20 - 1.21 \times X_1 - 0.23 \times X_2+0.83 \times X_3+0.038 \times X_1{X}_2+0.22 \times X_1{X}_3\\ - 0.12 \times X_2{X}_3+0.59 \times {X_1}^2 - 0.81 \times {X_2}^2 - 0.074 \times {X_3}^2\end{array}\right.}_{ }$$

(10)

where, F is the collision impact force, N, and X₁, X₂, and X₃ are the plug specification, contact material and drop height (mm), respectively.

The analysis of variance (ANOVA) and significance were carried out on the quadratic regression model of the collision impact force, and the results are shown in

Table 4. The analysis of variance (ANOVA) on the collision impact force.

Source	Sum of Squares	Freedom	Mean Square	F Value	p Value
Model	21.80	9	2.42	62.21	<0.0001 **
X1	11.62	1	11.62	298.34	<0.0001 **
X2	0.42	1	0.42	10.87	0.0132 *
X3	5.45	1	5.45	139.85	<0.0001 **
X1 X2	0.005625	1	0.005625	0.14	0.7151
X1 X3	0.20	1	0.20	5.09	0.0587
X2 X3	0.055	1	0.055	1.42	0.2725
X12	1.47	1	1.47	37.80	0.0005 **
X22	2.75	1	2.75	70.73	<0.0001 **
X32	0.023	1	0.023	0.59	0.4682
Residual	0.27	7	0.039
Lack of Fit	0.085	3	0.028	0.6	0.6484
Pure Error	0.19	4	0.047
Cor Total	22.07	16
R2	0.9877

Note: * indicates significant (p < 0.05), ** indicates highly significant (p < 0.01).

. The regression model of collision impact force was p < 0.0001, indicating that the model was extremely significant. The regression equation determination coefficient R² = 0.9877 and the adjusted determination coefficient adjusted R² = 0.9718 both were close to 1. Therefore, the regression equation had high reliability. The lack of fit p = 0.6484 > 0.05 indicated that the regression equation fit well and could better reflect the relationship between the collision impact force F and X₁, X₂ and X₃. In this model, X₁, X₃, X₁², and X₂² had very significant impacts on the collision impact force, and X² had a significant effect on the collision impact force. The influence on the collision impact force was on the order of plug specification > drop height > contact material.

3.3. Analysis of Collision Contact Stress Distribution between Oil Sunflower Plug Seedlings and Contact Materials

3.3.1. Collision Contact Stress Distribution Characteristics of Contact Material to Plug Seedlings

According to the analysis of the orthogonal test results, the contact material and the drop height had a significant effect on the collision impact force. A drop impact test with oil sunflower plug seedlings and three contact materials was carried out, and the contact stress and distribution of the seedling were obtained through the flexible film network tactile pressure sensor and the I-Scan System data processing system. The results are shown in Figure 8. When the oil sunflower seedlings were dropped and collided with different contact materials, a contact stress of 10–30 kPa accounted for the main area, which played a significant role in the pot damage. A contact stress of 30–35 kPa occurred at higher drop heights. Combined with the overall structure of the transplanter and the assembly relationship between various components, the drop height was less than 350 mm during seedling picking, feeding, carrying, and planting. Therefore, contact stress was not the main stress that caused pot damage. When the contact stress was 10~15 kPa, the contact area showed little difference at different drop heights. When the contact stress was >15 kPa, the contact area with a drop height of 350 mm was much larger than that of other drop heights. This shows that with the increase in the drop height, the high stress area accounts for the main contact area, which is the main reason for the collision damage of these pots.

The stress distribution of different contact materials after collision is shown in Figure 9. The contours of the seedlings colliding with different contact materials were in a relatively standard shape, and the contact area between the seedlings and the soil was significantly larger than that of other two materials. The dark blue area is low stress, the red area is high stress, and the colorless area has a contact stress of 0 kPa. When the drop height was 50 mm, the low-stress area occupied the main area. When the drop height was greater than or equal to 150 mm, the high-stress area was generally located in the middle of the contact area. With the increase in the drop height, the low-stress-distribution area decreased significantly, the high-stress area showed a gradually increasing trend, and the pot damage was larger.

3.3.2. Average Contact Stress and Contact Area of the Seedlings

As shown in Figure 10, as the drop height increased, the contact areas were linearly related between different contact materials, and the coefficients of determination (R²) were all greater than 0.95. When the drop height was constant, the collision contact area between the oil sunflower plug seedlings and the contact materials ranged from large to small: soil, steel, and ABS plastic. The soil was compacted before the test in the test soil bin, and there were still many pores in the soil. During the test, when the oil sunflower seedlings collided with the soil, the contact area was higher, and the matrix loss of the oil sunflower seedlings and the deformation of the pot were reduced. When the plug seedlings collided with the steel plate, the plastic deformation of the steel plate was small, and the deformation was mainly concentrated on the pot, which increased the pot damage and matrix loss. When the oil sunflower seedlings collided with the ABS plastic, the contact area was small, and the plastic had a certain elasticity, which buffered the collision and deformation of the seedlings. The variation mechanisms of the contact area of oilseed sunflower plug seedlings on different contact materials were different, indicating that there was a certain relationship between the contact area and pot damage.

As shown in Figure 11, the average contact stress between the plug seedlings and the steel plate was relatively large, ranging from 19.4 to 22.8 kPa, and the average contact stress tended to increase significantly with increasing drop height. The contact stress between the seedlings and the soil was in the middle level, ranging from 19 to 22 kPa, and the average contact stress fluctuated slightly. The average contact stress of the seedlings on the ABS plastic was significantly smaller than that on the steel plate and the soil. With increasing drop height, there was an obvious increasing trend, and the average stress range was 18.8~21 kPa. The variation mechanisms of the average contact stress of oil sunflower plug seedlings on different contact materials were different, indicating that there was a certain relationship between the average contact stress and pot damage.

3.3.3. Relationship between the Contact Stress Distribution of the Seedlings and Pot Damage

The relationship between the pot damage and the collision impact force between the oil sunflower plug seedlings and different materials is shown in

Table 5. Relationship between matrix loss and collision impact force of plug seedlings with different contact materials.

Contact Material	Regression Equation	R2
Steel	K = 1.81 × P × A × 10−3 − 8.13	0.976
Soil	K = 1.22 × P × A × 10−3 − 5.98	0.965
ABS Plastic	K = 2.11 × P × A × 10−3 − 9.43	0.940

Note: K is the matrix loss rate, %; P is the average contact stress, kPa; A is the contact area, mm².

. The collision impact force of different materials and the matrix loss rate of seedlings were linearly correlated, and the determination coefficients (R²) were all greater than 0.90. This shows that the contact stress and contact area of the plug seedlings with different materials measured by the flexible film network tactile pressure sensor could more accurately represent the pot damage.

3.4. Analysis of Collision Contact Stress Distribution between Oil Sunflower Plug Seedlings and Plug Specifications

3.4.1. Distribution Characteristics of the Collision Contact Stress between the Steel Plate and the Plug Seedlings with Different Plug Specifications

The seedling age is 30 days, and steel was selected as the contact material. The drop impact test of oil sunflower plug seedlings of different plug specifications was carried out. During the test, the contact stress distribution of the seedlings was obtained through the flexible film network tactile pressure sensor. The test results are shown in Figure 12. When the drop height was constant, the contact area of the oil sunflower plug seedlings of different plug specifications and the steel plate was, from small to large, 105 plug, 72 plug, and 50 plug. With the increase in the drop height, the low-stress-distribution area decreased significantly, the high-stress area showed a gradually increasing trend, and the pot damage was larger.

3.4.2. Average Contact Stress and Contact Area between the Steel Plate and the Plug Seedlings with Plug Specifications

As shown in Figure 13, the contact areas of drop impact of different plug specifications were linearly correlated, and the coefficients of determination (R²) were all greater than 0.90. When the oil sunflower plug seedlings dropped at the same height, the contact area of the oilseed sunflower seedlings of different plug specifications from large to small was as follows: 50 plug, 72 plug, and 105 plug. The main reason for this was that the plastic deformation of the steel plate was small, and the deformation was mainly concentrated on the pot. With increasing drop height, the contact area of the oil sunflower plug seedlings and the pot damage gradually increased. The changing mechanisms of the contact area between the oil sunflower plug seedlings of different plug specifications and the steel plate collision were different, resulting in completely different damage degrees of the pot.

As shown in Figure 14, the average contact stress of oil sunflower plug seedlings with different plug specifications and the steel plate was, from large to small, 105 plug, 72 plug, and 50 plug. The average contact stress between the 105 plug seedlings and the steel plate was significantly impact, and the average stress ranged from 18.1 to 21.9 kPa. The main reason for this was that the contact area of the 105 plug oil sunflower seedlings and the steel plate was small, and the stress was concentrated. The changing mechanisms of the average contact stress of oil sunflower seedlings under different sizes was different, indicating that there was a certain relationship between the average contact stress and pot damage.

3.4.3. Relationship between the Contact Stress Distribution of Plug Seedlings and Pot Damage

The relationship between pot damage and collision impact force for different plug specifications of oil sunflower seedlings with the steel plate is shown in

Table 6. The relationship between the matrix loss and the collision impact force of different specifications of oil sunflower plug seedlings.

Sizes	Regression Equation	R2
50 plug	K = 1.56 × P × A × 10−3 − 8.57	0.958
72 plug	K = 1.81 × P × A × 10−3 − 8.13	0.976
105 plug	K = 1.73 × P × A × 10−3 − 6.99	0.915

Note: K is the matrix loss rate, %; P is the average contact stress, kPa; A is the contact area, mm².

. The collision impact force of different specifications of oil sunflower plug seedlings was linearly correlated with the matrix loss rate of seedlings, and the coefficients of determination (R²) were all greater than 0.90. This shows that the contact stress distribution of different specifications of oil sunflower plug seedlings and the steel plate measured by the flexible film-network tactile pressure sensor could more accurately represent the pot damage.

4. Conclusions

In this paper, the kinetics equations of the plug seedlings were established in the process of dropping based on the generalized Hertz-theory. The drop impact test of the plug seedlings was carried out under different factors with the matrix loss rate as the test index. The Tekscan pressure distribution measurement system measured the collision impact force. The orthogonal tests were conducted under different factors. The influence degree of each factor on impact force was obtained. The regression equation between the contact stress distribution and the pot damage was obtained by analyzing the significant influence parameters. The main conclusions are as follows:

(1) The influence laws of different factors on pot damage were obtained through a drop impact test. The Tekscan pressure distribution measurement system measured the collision impact force, and the orthogonal tests were conducted. The test showed that the influence of the collision impact force was on the order of plug specification > drop height > contact material.

(2) The Tekscan pressure distribution measurement system measured the change law of contact stress distribution under significant influencing factors. The test results showed the collision contact area between the plug seedlings and different contact materials from large to small was soil, steel, and ABS plastic. The collision contact area between the plug seedlings and other plug specifications was 50 plug, 72 plug, and 105 plug from the largest to the smallest. When the plug seedlings collided with different contact materials, the average contact stress between the seedlings and the steel plate ranged from 19.4 kPa to 22.8 kPa. When the plug seedlings of various sizes collided with steel plates, the average contact stress was ordered as 105 plug, 72 plug, and 50 plug in descending order. A linear regression model between collision impact force and matrix loss rate under different factors was established based on the pressure data collected by the Tekscan pressure distribution testing system, and the determination coefficients (R2) were more significant than 0.90.