An ANSYS/LS-DYNA Simulation and Experimental Study of Sectional Hob Type Laver Harvesting Device

Abstract

To solve the problems of low net harvesting rate, high loss rate, and uneven stubble height during the harvest of laver, the laver (Porphyra yezoensis) was selected as the research object, the analysis of the cultivation mode, biomechanical characteristics, harvesting trajectory and force of laver were carried out. A sectional hob type harvesting device was designed. A rigid-flexible coupling model related to the interaction between the cutting mechanism and the laver was constructed based on ANSYS/LS-DYNA. The Box–Behnken design method was used to simulate the effects of different structural parameters and process parameters on the force of laver cutting, and the bench test of the laver harvesting device was carried out. The simulation results showed that the four factors that significantly affect the force exerted on the laver during cutting in proper order were cutter revolving speed, knife extension length, knife inclination angle and forward velocity. When the combination of the forward velocity, the cutter revolving speed, the knife extension length and inclination angle was 0.77 m/s, 900 r/min, 40 mm, and 110°, respectively, the cutting force on laver was the smallest, which was 4.21 N. The bench test of harvesting performance showed that the cutter revolving speed has a significant impact on the recovery rate, and the forward velocity has a significant impact on the loss rate. When the harvesting speed ratio was λ₄ (the cutter revolving speed was 900 r/min and the forward velocity was 0.77 m/s), the net harvesting rate and the loss rate were 97.45% and 3.38%, respectively, and the cutting proportion of laver can reach 77.5%. The results of the study provide a theoretical basis for the development of harvesting for laver.

1. Introduction

Laver (Porphyra yezoensis) is widely distributed in the world and is the main large economic algae cultivated in China [1,2]. China has a total yield of 2.22 × 10⁵ t in 2020, with the economic output value reaching CNY 18.3 billion [3]. Harvesting is a crucial link in the aquaculture of laver. Although some mechanical devices have been applied to laver harvesting in China, such as ZS160-1.8 laver harvester developed by Lianyungang Agricultural Machinery Factory [4,5,6], most of these mechanical devices are designed and manufactured by imitating foreign products. As a result, laver harvesting is limited by a heavy labor burden, low production efficiency, low net recovery rate, and high loss rate [7,8,9]. In contrast, some researchers from Japan, South Korea and other countries have designed some advanced laver harvesting devices, and they also conducted explorations into the interaction between harvesting cutters and laver, as well as the relationship between the cutter life and production efficiency. For example, Yada et al. [10] revealed that the closer the cutting point is to the root of laver, the greater the cutting stress on laver and the better the harvesting effect. Kim et al. [11] found that the longest service life of the cutter can be achieved under the traveling speed of 0.53 m/s and the rotation speed of 700~800 r/min, and the harvesting efficiency can reach 1.15~1.26 kg/min, thus realizing high-quality harvesting of laver [12,13,14,15,16]. However, laver is fixed on a single point and grows on the net curtain, due to the lack of systematic research on the force analysis and fracture state of laver during harvesting, it is difficult to conduct in-depth research on single point support cutting of laver, and the problem of high harvesting damage rate has not been effectively solved.

For the single point support cutting problem of flexible crops such as laver, by establishing a rigid flexible coupling model for simulation research, this model can be employed to effectively reduce the research difficulty, shorten the research cycle, and decrease the development costs [17]. Bílek and Kovařík [18] constructed a rigid-flexible coupling model for determining the parameters of wood sawing. They performed a stress analysis and found that the maximum stress caused by the temperature gradient was about ten times that inferred from the centrifugal force. This stress state was basically consistent with that obtained in a finite element model. Meng et al. [19] established a coupling model related to the interaction between the circular saw blade and branches of Morus alba. They simulated and analyzed the changes of sawtooth number, rotation speed, feeding speed, and branch diameter in the stress and cutting force in the branch-cutting of process. The bench test results verified that the simulated value of the cutting force was consistent with the measured value. Zhao [20] constructed a rigid-flexible “picking mechanism-fruit tree” coupling model based on ANSYS/LS-DYNA to simulate the dynamics of the vibration system. The vibration response characteristics of Camellia oleifera trees were also explored under different excitation frequencies and holding heights. The results proved that the excitation frequency of 38 Hz and the holding height of 1300 mm constituted the optimal harvesting parameter combination. Dun et al. [21] simulated the sawtooth rotary cutting process for the straw of Glycine max. They selected the cutting linear speed, cutter inclination angle, and operation speed as test factors, and analyzed the primary and secondary factors affecting cutting resistance and the optimal cutting parameters. Currently, the application of rigid-flexible coupling model research mainly focuses on the interaction between knives and materials with fixed form such as plant stems or wood, and the research and application of the interaction between knives and crops with greater flexibility such as laver has not been reported. The harvesting of laver involves a complex and random high-speed penetrating collision process. As the cutter and laver contact in a discontinuous dynamic mode, it would be impossible to observe and analyze the interaction between the cutter and laver by conventional methods. According to the crop-cutting theory [22], cutting will cause less damage to laver, while the rupture under a tensile force will cause more damage. The fester caused by damage is the main factor affecting the yield of laver.

Therefore, laver was selected as the research object in this study. The parameter combination for avoiding the missing cutting or re-cutting of laver was explored by analyzing the relationship between the inclination angle of knife and the stress on laver. A rigid-flexible coupling model related to the interaction between the cutting mechanism and the laver was constructed based on ANSYS/LS-DYNA, to perform a harvesting simulation. Moreover, the influence of different structures and operating parameters on the cutting force on laver was analyzed. After a laver harvester prototype was designed and manufactured, the simulation results were verified by a bench test. These efforts may provide a theoretical foundation for the research and development of laver harvesters.

2. Structure and Working Principle of the Harvester

2.1. Physical Property and Harvesting Requirements of Laver

Laver is mainly cultivated on raft net, the cultivation facilities include net curtains and floating rafts. Specifically, net curtains provide the substrate for the growth of laver seedlings, while floating rafts serve as the brackets for hanging these net curtains. The specification of a net curtain for the cultivation of laver is 1.5 m × 12 m [1,23]. Floating rafts can be divided into three patterns according to different cultivation conditions, namely full floating raft, pillar raft, and semi-floating raft (Figure 1) [24,25].

After laver grows to a certain length and the algal quality becomes poor, harvesting contributes to decreased losses timely and increased laver yields. During the harvesting of laver, it is required to keep the re-production ability of the laver. The laver can be harvested when it grows to 15–20 cm, the stubble length is generally 5–7 cm [26,27,28], and the low damage rate of laver shall be ensured during harvesting. The main biological characteristics of laver [29] are shown in

Table 1. The main physical parameters of laver.

Parameters	Values
Frond overall length (mm)	78.3~236.65
Frond head width (mm)	4.2~14.9
Frond middle width (mm)	10~21.4
Frond root width (mm)	8.8~19.6
Frond head thickness (mm)	0.013~0.028
Frond middle thickness (mm)	0.02~0.035
Frond root thickness (mm)	0.016~0.033
Density (wet) (g∙cm−3)	0.208~0.596
Density (dry) (g∙cm−3)	0.571~1.684

.

2.2. Structure and Principle of the Harvester

In this study, a laver harvester was designed based on the working environment and harvesting requirements. It was composed of a sectional hob mechanism, a stubble height adjustment mechanism, a net curtain support mechanism, a rack, and a hydraulic motor, as shown in Figure 2. Specifically, the sectional hob mechanism (length is 2000 mm) consisted of a knife shaft, 9 pairs of knives, and a connection support plate. The stubble height adjustment mechanism included an arc cover, a protective grid, and a rotary drum. The net curtain support mechanism contained a roller and a connector.

As shown in Figure 3, the laver is fixed on the net curtain at a single point. During the working of the laver harvester, the stubble length can be adjusted in advance by rotating the stubble height adjustment mechanism manually. The net curtain can be supported and flattened by the cooperation between the net curtain support mechanism and the arc cover. The laver can be cut by high-speed rotating knives during the forward movement of the loading device. Laver would fall into the collection box. The device controls the speed of the knife through the manual regulating flow valve, adjusts the speed to the required speed through the indication on the speed sensor, and realizes the emergency stop of the knife through the manual reversing valve.

3. Theoretical Analysis of the Harvesting Process

3.1. Knife Movement Trajectory Analysis

According to the working principle of the laver harvester, the movement trajectory of the knife top presents a trochoid curve combined by circular movement and horizontal movement [30]. The kinematic model of the knife top can be expressed as follows:

X = V₀t + Rsinωt

(1)

Y = −Rcosωt

(2)

where V₀ represents the traveling speed of the loading device (m/s), ω represents the angular speed of the cutter shaft (rad/s), t represents time (s), R represents the turning radius of the knife (mm).

The position of the knife top at any time can be expressed by Formulae (1) and (2). There are different shapes and characteristics of the movement trajectory with the changes in the traveling speed (V₀), turning radius of the knife (R), and angular speed (ω).

As shown in Figure 3, the speed of the knife top in the horizontal and vertical directions is the derivative of displacement (X) and (Y) to time (t):

$$V_X=\frac{d_X}{d_t}=V_0+R\omega \cos \omega t$$

(3)

$$V_Y=\frac{d_Y}{d_t}=R\omega \sin \omega t$$

(4)

As shown in Figure 4, due to the fact that the cutter can only contact laver in the solid line part during harvesting, only the movement trajectory of this part would be analyzed. The axle center of the cutter is point P₅, and the intersection point O of the net curtain and the Y axis is the origin. When the anterior knife top I contacts the laver at point P₁, the laver on the net curtain would be cut. Then, the knife top continues to move to the right side until the posterior knife top leaves the lowest pendulous point of the laver at point P₂. The range from point P₁ to point P₂ indicates the area for cutting laver. Point P₁ is the highest cutting point, and the rotation speed overlaps with the traveling speed, which induces the maximum cutting force on the laver. The distance from the net curtain to the cutting baseline represents the target stubble length (H₂), and the laver length is H₁. The movement trajectory of the posterior knife top II is the same as that of the anterior knife top I. Point P₄ represents the position entering the cutting area, the missing cutting length of laver is H₃, and the shaded area (P₁ P₃ P₄) represents the missing cutting area.

To avoid missing cutting, reducing the missing cutting length (H₃) can effectively reduce the missing cutting area (P₁ P₃ P₄), H₃ is related to the movement trajectory of the knife top and the cutting pitch. The cutting pitch (the distance from point P₁ to point P₃ in Figure 3) is the traveling distance of the harvester during the time interval for the cutting of laver by two adjacent cutters. If the time interval for the cutting of laver by two adjacent cutters is set as:

$$t_s=\frac{2\pi }{3\omega }$$

(5)

the cutting pitch can be expressed as:

$$S=V_0{t}_s=\frac{2\pi V_0}{3\omega }$$

(6)

if the ratio of the rotation speed of the cutter to the traveling speed λ is set as:

$$\lambda =\frac{R\omega }{V_0}$$

(7)

the approximate calculation can be completed as per the following formula:

$$\frac{S}{2}=\frac{a}{\omega }{V}_0+R\sin a$$

(8)

when the value of angle a is not large, it can be considered that sin a=a.

$$\frac{S}{2}=a\left(\frac{V_0}{\omega }+R\right)$$

(9)

$$a=\frac{S}{2\left(\frac{V_0}{\omega }+R\right)}=\frac{\frac{2\pi R}{3\lambda }}{2\left(\frac{R}{\lambda }+R\right)}=\frac{\pi }{3\left(\lambda +1\right)}$$

(10)

hence, the missing cut length can be simplified as:

$$H_3=R\left(1-\cos a\right)=R\left(1-\cos \frac{\pi }{3\left(\lambda +1\right)}\right)$$

(11)

where V₀ represents the traveling speed of the loading device (m/s), R represents the turning radius of the knife (mm).

Therefore, the traveling speed, the rotation speed of the cutter, and the turning radius of the knife directly affect the missing cutting length. When the turning radius is constant, the reducing speed ratio λ, the missing cutting length (H₃) will decrease, and the missing cutting area.

3.2. Force Analysis in the Cutting Process

When the knife top cuts into the laver, the force is shown in Figure 5.

Laver is selected as the research object, the X axis represents the traveling direction, the Y axis represents the pendulous direction of laver, F_X represents the horizontal component of the cutting force (F), and F_Y represents the vertical component of the cutting force (F), as expressed in Formulae (12) and (13).

F_X = Fcosθ + T − T_rsinθ_r

(12)

F_Y = Fsinθ + G − T_rcosθ_r

(13)

where F represents the cutting force of the cutter on laver (N), T represents the horizontal thrust of the knife on laver (the force on laver when the device moves forward) (N), T_r represents the self-elasticity of laver (N), G represents the weight of laver below the action point (N), θ represents the cutting rotation angle (°), θ_r represents the deflection angle of laver (°).

The horizontal component (F_X) is perpendicular to laver. One end of the laver is fixed, and the other end is unconstrained. If the weight of the laver is very small relative to the horizontal component, the laver may jump forward unsteadily, which may cause cutting failure. The weight (G) of the harvested laver will change with the movement of the action point of the knife. The closer to the root, the greater the weight (G) will be. Therefore, in order to cut off laver successfully, it is necessary to consider the weight of the laver below the action point.

The direction of the vertical component (F_Y) of laver is the same as that of the pendulous tension of laver. Some parts of the laver may be stretched and broken due to its soft texture, and hence there is a tensile force. The cutting stress and tensile stress on laver can be expressed as Formulae (14) and (15):

$$\tau =\frac{F\cos \theta +T-T_r\sin {\theta }_r}{A_s}$$

(14)

$$\sigma =\frac{F\sin \theta +T_r\cos {\theta }_r}{A_s}$$

(15)

where A_s represents the cross-sectional area of laver.

When $\tau >\sigma$, laver would be cut off; when $\tau <\sigma$, laver would be ruptured under a tensile force. If θ_r, F, and A_s in Formulae (14) and (15) are constant, $\tau$ and $\sigma$ would be proportional to the cutting rotation angle (θ), and laver may be cut off or ruptured under a tensile force with the changes in the cutting rotation angle. When the cutting rotation angle is zero, namely when the cutting edge acts vertically on laver, $\tau$ would reach the maximum, and laver can be harvested only by the cutting stress. If the cutting rotation angle increases, $\tau$ would decrease; otherwise, $\sigma$ would increase. However, when the cutting rotation angle is close to 90°, the cutter would not affect the harvesting of laver, it is impossible to complete the harvesting of laver. For rotary cutting, Li and Yada gave similar laws and conclusions [10,31]. In summary, when the cutting rotation angle decreases, the proportion of laver under a cutting force would increase; when the cutting rotation angle increases, the proportion of laver under a tensile force would increase.

The cutting rotation angle (θ) is determined by the knife position, knife structure parameters including extension length (B) and inclination angle (A), and operating parameters including rotation speed (D) and traveling speed (C). Under the same knife position and operating parameters, the cutting rotation angle (θ) is determined by cutter structure parameters. When the extension length (B) increases and the inclination angle (A) decreases, θ increases; otherwise, θ decreases. The single grid of the laver net curtain is 80 mm, and the length of the laver stubble is 50–70 mm. In comprehensive consideration of the structure size of the knife should not be too large and the stubble space requirement, the knife extension length is 35~45 mm. In order to make the top of knife better contact with the laver, the inclination angle is 100~120°. Based on the knife movement trajectory analysis results and the speed of the existing fishing boats, the traveling speed is determined to be 0.51~1.03 m/s, and the rotation speed is 700~1100 r/min.

4. Laver Cutting Simulation Study

A rigid-flexible coupling model related to the interaction between the cutting mechanism and the laver was constructed based on ANSYS/LS-DYNA. The laver model is endowed with flexible body features, and the knife is endowed with rigid body features. The laver model is calibrated by experiment and simulation, the error between the actual value and the simulated value of the fracture limit displacement is 3.75%, and the error between the actual value and the simulated value of the breaking force is 13%, which can be used for simulation research.

4.1. Model Establishment and Simulation Results

There are different lengths and multi-layer stacking in the laver. To realize simplification, the bending caused by stacking may be ignored, the laver model (length is 120 mm, width is 12 mm, thickness is 0.13 mm) was simplified to an inverted conical strip, and the model parameters [29] are shown in

Table 2. Material parameters of model.

Parameter	Laver [29]	Cutter
Poisson’s ratio	0.33	0.33
Shear modulus (Pa)	3.26 × 107	-
Elastic modulus (Pa)	2.54 × 106	2.00 × 1011
Tensile strength (MPa)	0.95	-
Shear strength (MPa)	2.33	-
Density (kg·m−3)	0.96	7850

. According to the actual cultivation pattern of laver, the root of laver was fixed. The interference of external factors in the laver movement was also ignored. Meanwhile, to shorten the simulation time, the turning radius of the cutter (111 mm) remained unchanged. Further, the axial length of the cutter was shortened and simplified to a one section hob (length is 667 mm), and the vibration factors in actual work were also ignored. During model establishment, the knife shall be as close to the laver as possible. Moreover, an explicit dynamic analysis was performed on the finite element model of the cutter and laver in the harvesting process by the coupling solution method.

The model related to cutter and laver was established by Solidworks, it was imported into ANSYS/LS-DYNA. The rigid setting was selected for the cutter, while the flexible setting was selected for the laver with the same characteristics. After the laver slices were finely divided, the mesh size was 2 mm, and a total of 15,863 meshes were obtained (Figure 6).

The total simulation time was set as 0.012 s, the root of laver was fixed and restrained, and the linear traveling speed and rotation speed of the cutter were set as 0.77 m/s and 900 r/min, respectively. The simulated harvesting process is shown in Figure 7.

After the Body contact tracker command was added for the laver model slices, the stress curves of the laver model slices in the X and Y axes during harvesting were obtained, as shown in Figure 8a,b, respectively. The time required for cutting laver was 0.005 s. Under the action of the knife on laver, the force on laver in the X and Y axes reached a peak value and decreased rapidly with the rupture of laver. The peak value in the X and Y axes was 4.56 N and 1.52 N, respectively (no force was generated in the Z axis). The peak value of the resultant force on the laver reached 4.8 N after the force combination.

4.2. Response Surface Test Design

In this study, the rotation speed, traveling speed, inclination angle, and extension length of the knife were selected as influencing factors. The cutting force on the laver was selected as an evaluation index to perform a response surface test by the Box–Behnken method [31]. The codes of each factor level in the test are listed in

Table 3. Level of test factors for response surface.

Level	Factor
	Inclination Angle A (°)	Extension Length B (mm)	Traveling Speed C (m∙s−1)	Rotation Speed D (r∙min−1)
−1	100	35	0.51	700
0	110	40	0.77	900
1	120	45	1.03	1100

.

4.3. Test Results and Analysis

The test scheme and results are listed in

Table 4. Level of test factors for response surface.

Test No.	Inclination Angle A (°)	Extension Length B (mm)	Traveling Speed C (m∙s−1)	Rotation Speed D (r∙min−1)	Cutting Force (N)
1	110.00	40.00	0.77	900.00	4.96
2	110.00	35.00	0.51	900.00	4.66
3	110.00	40.00	0.77	900.00	5.41
4	100.00	45.00	0.77	900.00	5.04
5	110.00	45.00	0.51	900.00	4.42
6	120.00	45.00	0.77	900.00	6.64
7	100.00	40.00	0.77	1100.00	6.00
8	100.00	40.00	0.51	900.00	4.24
9	110.00	45.00	0.77	1100.00	5.48
10	110.00	35.00	0.77	1100.00	5.88
11	110.00	40.00	0.77	900.00	5.16
12	120.00	40.00	0.77	1100.00	8.13
13	110.00	45.00	1.03	900.00	6.14
14	120.00	40.00	0.77	700.00	4.17
15	110.00	40.00	0.77	900.00	5.00
16	110.00	35.00	1.03	900.00	3.31
17	110.00	40.00	1.03	700.00	2.79
18	120.00	40.00	1.03	900.00	3.89
19	110.00	35.00	0.77	700.00	3.19
20	120.00	40.00	0.51	900.00	4.90
21	110.00	40.00	0.51	700.00	3.16
22	100.00	40.00	1.03	900.00	4.34
23	110.00	40.00	0.77	900.00	4.21
24	100.00	40.00	0.77	700.00	3.34
25	120.00	35.00	0.77	900.00	3.77
26	100.00	35.00	0.77	900.00	5.43
27	110.00	45.00	0.77	700.00	3.63
28	110.00	40.00	1.03	1100.00	5.83
29	110.00	40.00	0.51	1100.00	5.45

. A total of 29 groups of tests were performed, each group of tests was repeated three repeated times. The regression analysis and factor variance analysis were performed with Design-Expert 8.0.6. The significance of each factor on indexes was analyzed, and the regression model of the cutting force on laver was established.

Based on multiple regression fitting of the test data, a regression model established for the influence of various test factors on the cutting force (Y) on laver:

Y = 4.95 + 0.43B + 1.37D + 0.81AB + 0.77BC − 0.58C²

(16)

the variance analysis results of response surface tests (

Table 5. ANOVA results for response surface tests.

Index	Source	Squares	df	Mean Square	F	p
Y	Model	34.86	14	2.49	6.52	0.0006 **
	A	0.81	1	0.81	2.11	0.1684
	B	2.18	1	2.18	5.69	0.0317 *
	C	0.023	1	0.023	0.061	0.8081
	D	22.66	1	22.66	59.30	<0.0001 **
	AB	2.66	1	2.66	6.95	0.0195 *
	AC	0.31	1	0.31	0.81	0.3845
	AD	0.42	1	0.42	1.11	0.3108
	BC	2.36	1	2.36	6.17	0.0263 *
	BD	0.18	1	0.18	0.46	0.5079
	CD	0.14	1	0.14	0.37	0.5538
	A2	0.47	1	0.47	1.23	0.2858
	B2	0.002	1	0.002	0.006	0.9414
	C2	2.15	1	2.15	5.63	0.0326 *
	D2	0.048	1	0.048	0.12	0.7294
	Residual error	5.35	14	0.38
	Lack of fit	4.54	10	0.45	2.26	0.2250
	Error	0.81	4	0.20	Not significant
	Sum	40.21	28

Note: p < 0.01 (Extremely significant, **); p < 0.05 (Significant, *), the same below.

) indicated that there was a statistical significance in the model (p = 0.0006). The lack of fit of the regression model was not significant (p > 0.05), which indicated that the regression model had a high fitting degree, and the p-value of D was less than 0.01, which indicated that this term had a significant influence on the cutting force on the laver. The p-value of B, AB, BC, and C² was less than 0.05, which indicated that the above four items had a significant influence on the cutting force on laver. Further, there was a quadratic relationship for the influence of related test factors on the response value. The four factors with the most significant influence on the cutting force on the laver can be ranked as the rotation speed (D), extension length (B), inclination angle (A), and traveling speed (C).

As shown in Figure 9a, the interaction between the extension length and the inclination angle of knife can exert a significant influence on the cutting force of laver. The extension length of knife has a larger influence than the inclination angle. The cutting force would increase with the increase in the extension length, and it would decrease first and then increase more significantly with the increase in the inclination angle. The main reason is that the extension length increases the knife torque, which leads to the increase in cutting force on laver. At the same time, the knife inclination angle increases, the cutting force would decrease first and then increase, and the increase is not obvious. When the extension length ranges from 35 mm to 40 mm and the inclination angle ranges from 105° to 115°, the cutting force would be relatively small. As shown in Figure 9b, the extension length exerts a more significant influence on the cutting force compared with the traveling speed. The cutting force increases with the extension length, and it would increase first and then decrease with the increase in the traveling speed. The main reason is that the traveling speed increases, the speed variation increases, and the impact force on the laver gradually increases. At the same time, with the increase in traveling speed, the cross-sectional area changes with the position of the cutting point of the knife. With the increase of traveling speed, the cutting point of the knife moves close to the root of the laver and then moves away from the root of the laver, and the cross-sectional area of the laver first increases and then decreases, resulting in an increase in the cutting force on laver first and then decreases. When the extension length ranges from 35 mm to 40 mm and the traveling speed ranges from 0.77 m/s to 1.03 m/s, the cutting force would be relatively small.

4.4. Parameter Optimization

Taking the minimum cutting force was selected as the optimization objective, the objective function can be expressed as follows:

$$\{\begin{array}{c}minY\left(A,B,C,D\right)\\ 100°\le A\le 120°\\ 35\mathrm{mm}\le B\le 45\mathrm{mm}\\ 0.51\mathrm{m}/\mathrm{s}\le C\le 1.03\mathrm{m}/\mathrm{s}\\ 700\mathrm{r}/\min \le D\le 1100\mathrm{r}/\min \end{array}$$

(17)

After optimization, the optimum parameter combination can be obtained. The minimum cutting force was 4.21 N under an inclination angle of 110° (A), an extension length of 40 mm (B), a traveling speed of 0.77 m/s (C), and a rotation speed of 900 r/min (D), thus achieving the optimal cutting effect. The optimal structure and operating parameter combination optimized by simulation tests provide a basis for the subsequent bench test.

5. Bench Test

5.1. Test Materials and Methods

The net curtain of laver was collected from Zhuanghe sea area (Dalian, China). The size of the net curtain was modified to 1.5 m × 1.2 m. The net curtain was soaked in seawater before the bench test.

5.1.1. Harvesting Bench Test

As shown in Figure 10, the test bench was mainly composed of a laver-harvesting device (3), a net curtain (1), and net curtain traveling simulation devices (2 and 4). The harvesting device remained fixed during the test. One side of the net curtain passed through the opposite roller mechanism, and the other side was connected with the hexagonal roller shaft through the upper part of the harvesting device. The rotation speed of the cutter was controlled by a throttle valve, and the net curtain was driven to move in translation by a hexagonal roller shaft, and speed adjusted by a frequency converter to simulate the relative motion of the cutter and the curtain.

5.1.2. Cutting Fracture Detection Test

To explore the rupture state of laver under different operating parameters of the harvester and determine the cutting and rupture proportions, a sampling inspection was performed. The rupture state was predicted by macroscopic observation and then judged by observing the cells at the rupture edge under a microscope. Three net curtains (the net curtain harvested by speed ratio λ₃, λ₄, and λ₆) were selected for each group of tests, and 40 pieces of stubble laver were selected from each net curtain according to the five-point sampling method [32].

The test factors and levels are listed in

Table 6. Bench test factors and levels.

Test No.	Rotation Speed (r∙min−1)	Traveling Speed (m∙s−1)	Speed Ratio (λ)
1	800	0.51	λ1
2	900		λ2
3	1000		λ3
4	900	0.77	λ4
5		1.03	λ5
6	800	1.03	λ6

. As listed in the table, the speed ratio (λ) refers to the ratio of the rotation speed to the traveling speed, and λ_1–6 represents different speed ratios. During the bench test, the speed ratio test of each group was repeated three times.

5.2. Test Evaluation Index

According to the simulation results, the inclination angle of the cutter was 110°, and the extension length was 40 mm. The stubble height was adjusted to 50 mm in the bench test, and the harvested laver was collected and weighed after the test. Additionally, the traveling speed and rotation speed were selected as test factors, and the net recovery rate and loss rate were selected as evaluation indexes.

The evaluation of the net recovery rate is shown in Figure 11, which can be calculated as follows:

$$y_1=\frac{n_1}{n_1+n_2}\times 100\%$$

(18)

where n₁ represents the weight of the harvested laver (kg), n₂ represents the weight of the laver with an ineligible stubble length on the net curtain (kg).

The evaluation of the loss rate is shown in Figure 12. After the laver harvesting ship was simulated, the laver falling outside the specified area (outside the ship) was regarded as the loss amount, which can be calculated as follows:

$$y_2=\frac{m_1}{m_1+m_2}\times 100\%$$

(19)

where m₁ represents the weight of the harvested lever outside the specified area (kg), m₂ represents the weight of the harvested lever within the specified area (kg).

Determine the proportion of laver cutting and pulling according to the fracture morphology. Specifically, laver with a flat fracture can be classified into the cutting group (left side of Figure 13); while laver with a serrated fracture can be classified into the rupture group (middle part of Figure 13). As for laver with the two states (right side of Figure 13), the mesh in the figure can be selected as a classifying standard (each mesh was a square with a size of 5 × 5 mm). When the horizontal length of the fracture is larger than 2/3 of the whole length, it can be classified into the cutting group; otherwise, it can be classified into the rupture group.

The cell arrangement at the fracture of laver can be magnified 4 times or 10 times under the microscope with the assistance of TCapture 5.1.1 software. Specifically, laver with a relatively flat fracture and properly arranged marginal cells can be classified into the cutting group, while laver with an uneven fracture and cell fluid exudation can be classified into the rupture group.

5.3. Test Results and Analysis

5.3.1. Influence of the Rotation Speed on Harvesting Effects

As shown in Figure 14, under three tests (No.1, 2, and 3), the net recovery rate of laver increases first and then remains stable with the increase of the rotation speed. The loss rate increases with the increase of the rotation speed. After the rotation speed of the cutter increases from 800 r/min to 1000 r/min, the net recovery rate and loss rate of laver increases by 9.37% and 2.23%, respectively. With the increase in the rotation speed, the missing cutting area along the cutting trajectory decreases, and the relative cutting area and the net recovery rate increase. Due to the viscoelastic feature, laver may undergo plastic deformation before the fracture. With the increase in the rotation speed, the plastic deformation time would shorten, and the impact of cutters on laver would be enhanced. The higher splash weight of laver would also increase the loss rate. When the rotation speed of the cutter increases from 900 r/min to 1000 r/min, the net recovery rate only increases by 0.93%, which is not significant. In addition, the loss rate increases by 1.71%. This can be mainly explained that when the traveling speed is constant, the missing cutting area decreases and gradually remains stable after the rotation speed reaches a certain value. Meanwhile, the high rotation speed could exert a great impact on the laver, the throwing speed would also increase, and the splash weight outside the collection area would increase. Therefore, an appropriate rotation speed can improve the net recovery rate, avoid a larger loss rate, and reduce unnecessary energy consumption.

5.3.2. Influence of the Traveling Speed on Harvesting Effects

As shown in Figure 15, under three tests (No.2, 4, and 5), the net recovery rate gradually decreases with the increase in the traveling speed, while the loss rate increases sharply after a slow increase with the increase in the traveling speed. After the traveling speed increases from 0.51 m/s to 1.03 m/s, the net recovery rate decreases by 2.44%, and the loss rate increases by 4.02%. This can be mainly explained that the missing cutting area along the cutting trajectory increases with the increase in the traveling speed. When the cutting area decreases, the number of laver entering the cutting area increases per unit time. The cutting effect on the back row of the laver in the cutting area becomes worse, and incomplete cutting may be induced. Moreover, a higher traveling speed also contributes to a higher throwing speed. As a result, the net recovery rate is relatively reduced, and the loss rate is relatively increased.

5.3.3. Influence of Speed Ratio on the Proportion of Cutting and Pulling

The microscopic observation results related to fractures are shown in Figure 16. The laver harvested from cutting presents a flat fracture and properly arranged marginal cells (Figure 16a). The laver harvested from ruptures presents cell fluid exudation at the fracture and improperly arranged marginal cells (Figure 16b) The laver with a cutting-ruptured fracture only presents improperly arranged cells around the rupture (Figure 16c).

Figure 17 presents the statistical results of the microscopic examination of laver fractures under different speed ratios. When the speed ratios (λ₃, λ₄, and λ₆) of the three groups changes from large to small, the proportion of harvested laver increases significantly. This may be explained that when the speed ratio is large, the downward vertical speed of the cutter is larger, which makes a great effective contribution to the tensile speed. Hence, the proportion of harvested laver is larger when the speed ratio is small. When the speed ratio is small, the forward horizontal speed of the cutter is larger, which has a great effective contribution to the cutting speed, thus realizing effective cutting. Hence, the proportion of harvested laver increases. After the net recovery rate and loss rate are compared under different operating parameters, the speed ratio (λ₄) can be identified to ensure an appropriate net recovery rate and a relatively small loss rate, the laver harvester can achieve a favorable cutting effect.

6. Conclusions

In this study, a laver harvester was designed and manufactured. The relationship between the movement trajectory and the inclination angle of cutters and the stress on the laver was clarified. A rigid-flexible coupling model related to the interaction between the cutting mechanism and the laver was constructed. The Box–Behnken design method was used to simulate the effects of different structural parameters and process parameters on the force of laver cutting. In addition, the bench test and cutting fracture morphological microscope observation of laver harvesting device were carried out.

(1) The results indicated that the inclination angle of the knife was inversely proportional to the cutting stress and directly proportional to the tensile stress. When the inclination angle decreased, the cutting proportion increased; otherwise, the rupture proportion increased. The order of factors affecting the force of laver cutting was cutter revolving speed, knife extension length, knife inclination angle, and traveling velocity. It was confirmed that the cutting force on laver was 4.21 N under the optimal parameter combination with a traveling speed of 0.77 m/s, a rotation speed of 900 r/min, an extension length of 40 mm, and an inclination angle of 110°. The bench test under this parameter, the net recovery rate and the loss rate were 97.45% and 3.38%, respectively, the cutting proportion of laver can reach 77.5%. This result further validated that an excellent harvesting effect can be achieved under this parameter combination.

(2) According to the simulation results, the force required to harvest laver is 4.21~8.13 N, and it is recommended that the force in the actual harvesting process should not be lower than this range. At the same time, it is particularly important to match the traveling speed with the rotation speed of the device in the complex marine environment, in order to the harvesting process needs to have a low loss rate, damage rate, and a high net recovery rate. It is recommended that the traveling speed of the device and tool speed be 0.77 m/s and 900 r/min. This study provides reference for the research methods of flexible crops such as laver.