Design and Experimental Research on an Automated Force-Measuring Device for Plug Seedling Extraction

Abstract

Existing force-measuring devices lack versatility in studying the dynamic coupling process between the seedling-picking device and the plug seedling pot during automatic transplanting. This research developed a universal force-measuring device featuring a centrally symmetrical clamping needle layout and a simultaneous insertion and clamping mechanism. The force-measuring device enables the flexible adjustment of the number of clamping needles (2/3/4 needles) via a modular structure. It can also modify the insertion depth and angle of the clamping needles to accommodate three specifications of plug seedlings, namely 50-hole, 72-hole, and 128-hole plug seedlings. A real-time monitoring system with dual pull-pressure sensors is integrated to precisely acquire the dynamic response curves of the clamping force (F_J) and the disengaging force (F_N) of the plug seedling pot during the seedling-picking process. Taking water spinach plug seedlings as the research object and combining with EDEM-RecurDyn coupling simulation, the interaction mechanism between the clamping needle and the plug seedling pot was elucidated. The performance of the force-measuring device was verified through systematic force-measuring experiments. The main research findings are as follows: The force-measuring device designed in this study can successfully obtain the mechanical characteristic curve of the relevant seedling plug pot throughout the automatic seedling-picking process. The simulation results show high consistency with the experimental results, indicating that the force-measuring device can effectively reveal the dynamic coupling process between the seedling-picking device and the plug seedling pot. The verification experiment demonstrates that the force-measuring device can effectively quantify the mechanical properties of the of plug seedling pots under different plug seedlings specifications and different clamping needles configurations. Reducing the hole size and increasing the number of clamping needles can effectively decrease the peak value of the disengaging force (F_Nmax). The peak clamping force (F_Jmax) is approximately inversely proportional to the needle number, with the four-needle layout providing the most uniform force distribution. The force-measuring device developed in this study is functional, applicable, and versatile, offering a general force-measuring tool and a theoretical foundation for optimal seedling-picking device design.

1. Introduction

Transplanting plug seedlings to the field enhances crop resistance to drought, flood, and salinity. This practice also extends the effective growth cycle. As a result, crops grow more steadily, which improves yield and quality [1,2,3,4,5]. During the process, the transplanting machine is the most important tool. Moreover, the technology of fully automatic transplanting machines has become relatively mature in developed countries such as Europe and America [6]. With the development of intelligent technologies, precision agriculture, and innovative techniques, significant progress has been made in transplanting machines to meet the low-cost and simple automation needs in developing countries [7]. As a crucial component of the transplanting machine, the seedling-picking device is the key to replacing manual labor and realizing automatic transplanting technology. It directly determines the quality and efficiency of the transplanting machine industry [5,8]. The seedling-picking mechanism in automatic transplanters can repeatedly extract plug seedlings automatically from the seedling plugs with the help of a pair of pins or forks, which are designed to gently grip the seedlings without causing damage, and drop them at a predefined location [9]. During the seedling-picking process in the current plug-seedling transplanting operation, there exists an issue where the seedling-picking device is unstable and prone to damaging plug seedlings, which reduces the survival rate of plug seedlings after transplantation [10,11].

At present, in standard Chinese automatic transplanting machines, the operation mode of the seedling-picking device is usually to insert the clamping needles in the seedling-picking end-effector into the plug seedling pot, applying an appropriate clamping force to grip the plug seedling pot for seedling-picking, and then depositing the seedling pot into the planting device to complete the seedling-throwing process [12]. Therefore, this method of seeding-picking by inserting clamping needles into the plug seedling pot has received extensive attention. Therefore, as a component that directly acts on the plug seedling pot, the seedling-picking device needs to grip the plug seedling pot from the tray stably. Meanwhile, it should also ensure that the clamping force applied to the plug seedling pot does not cause damage to the pot [13,14]. Hence, it is necessary to quantitatively explore the interaction between the seedling-picking device and the plug seedlings.

Previously, a large number of scholars have conducted extensive research on the interaction between the seedling-picking device and the plug seedling. Yang et al. [15] conducted a study on the performance-influencing factors of the sliding needles equipped with sensors during the process of robot-assisted plug seedling transplantation. These factors encompass mechanical aspects, including the clamping angle, extraction acceleration, and sensor sensitivity, as well as horticultural elements such as plant species, the adhesion between roots and hole walls, and the substrate moisture content. Tong et al. [16] designed a mechanical experimental platform founded on a universal testing machine, which is capable of adjusting the outer surface of the pointer clamping for compressing the plug seedling pot. Via the experiment, the optimal combination was acquired, specifically a pointer clamping angle of 7° and a quantity of four clamping needles. Under this condition, a clamping force exceeding 2.6 N can be achieved for the horizontal extrusion of a 4 mm plug seedling pot, thereby surmounting the gravitational force of the plug seedling and the adsorption force between the plug seedling and the seedling plug. Jiang et al. [17] designed and verified a four-needle end-effector that combines the advantages of clamping and sliding. The interaction between the extrusion force F_K and the adhesion force F_L during the clamping process and its influence on the transplanting performance were analyzed. The results show that when the F_K/F_L ratio was in the range of 5.99~8.67, the performance of the end-effector was optimal. Kang et al. [18] developed a two-row automatic vegetable transplanter suitable for small-scale farmland in South Korea, focusing on comparing the performance differences between finger-type and fork-type seedling-picking devices to determine a better finger-type seedling-picking device. Jin et al. [19] designed an integrated sensor for measuring the clamping force of seedling-picking claws of vegetable plug seedlings, which realized the integrated design of seedling-picking claws and sensors, and could be used for the real-time and accurate detection of clamping force changes during transplanting. Tian et al. [20] simulated the interaction between the clamping needle and the plug seedling pot using EDEM discrete element simulation. They optimized the diameter of the clamping needle, the depth of insertion into the plug seedling pot and the speed of grasping the plug seedling pot by combining the plug seedling pot damage rate with the response surface method. Han et al. [21] designed a multi-needle clamp-type seedling-picking mechanical experimental bench based on the two-finger four-needle structure of the seedling-picking device. The cylinder drives the clamping needles to complete the oblique insertion, parallel clamping and vertical extraction of the clamping needle. They investigated the impact of the clamping needle diameter and seedling-picking speed on the mechanical properties of the seedling-picking process. This was achieved by combining a displacement sensor and a pull-pressure sensor to measure the force. Magar et al. [22] developed and optimized an embedded system-based automatic seedling-picking device for vegetable plug seedlings. They optimized the main influencing factors such as crank rotational speed, seedling root bulb moisture content and needle penetration depth in the root ball, to maximize the extraction ratio and minimize the failure ratio via the response surface method.

Nevertheless, notable limitations persist in the extant research. Currently, there is a scarcity of relevant force-measuring devices employed to investigate the mechanical properties of plug seedling pots during the automatic seedling-picking process. Moreover, their design lacks universality, making it challenging for the existing force-measuring devices to conduct automatic seedling-picking force-measuring experiments for diverse experimental objectives. In particular, the seedling-picking method featuring a centrally symmetrical clamping needle layout and a simultaneous insertion and clamping mechanism can mitigate the disturbance and damage to the plug seedling pot matrix soil [23]. Research on this type of seedling-picking method has garnered extensive attention.

In light of the aforementioned issues, this study presents a universal automatic seedling-picking force-measuring device founded on the centrally symmetrical clamping needle layout and the simultaneous insertion and clamping mechanism. The force-measuring device enables the flexible alteration of the number of clamping needles (2/3/4 needles) via the modular structure. It can regulate the insertion depth and angle to accommodate the three specifications of 50-hole, 72-hole, and 128-hole plug seedlings. The integrated dual pull-pressure sensor real-time monitoring system can precisely acquire the mechanical curve of the seedling plug during the seedling-picking process. Secondly, we chose four clamping needles to conduct an automatic seedling-picking coupling simulation experiment for 50-hole plug seedlings. We analyzed the interaction between the clamping needles and the plug seedling pot particle model through EDEM-RecurDyn coupling simulation. Furthermore, we verified the performance of the force-measuring device via a systematic force-measuring experiment (covering different plug seedlings specifications and different numbers of clamping needles). Finally, an experiment to measure force during was conducted using the force-measuring device to evaluate its functionality. The results of the coupling simulation experiment were compared with those of the corresponding mechanical experiment to confirm the applicability of the force-measuring device. The experimental outcomes with various plug seedlings specifications and different numbers of clamping needles were analyzed, and the mechanical properties of plug seedlings under the experimental condition were quantified to verify the universality of the force-measuring device. The objective of this study is to propose an automated force-measuring device for plug seedling extraction.

2. Materials and Methods

2.1. Design of Force-Measuring Device

2.1.1. The Overall Structure of the Force-Measuring Device

The force-measuring device described in this study has been fully developed, including detailed design, physical construction, and experimental prototyping. All figures (Figure 1, Figure 2 and Figure 3) depict the actual device built and tested in this research. Permission for publication of the device design has been obtained.

The force-measuring device designed in this study is composed of the seedling-picking force-measuring component, seedling plug positioning plate, connecting plate, lifting module, proximity switch, linear guide, slider, connecting block, control cabinet, frame and so on (Figure 1a). The seedling-picking force-measuring component is secured to the lifting module via the slider. The slider and the linear guide rail play a guiding role in the seedling-picking force-measuring component, eliminating the influence of the shaking of the mechanism during the force-measuring process.

The designed force-measuring device quantitatively detects the interaction between the seedling-picking force-measuring component and the plug seedlings during the automatic seedling-picking process (Figure 1b). The working process of the force-measuring device for seedling-picking operation is as follows: The plug seedlings are placed on the seeding plug positioning plate, and the lifting module drives the seedling-picking force-measuring component to drop from the zero height to the seedling-picking point. After that, the seedling-picking force-measuring component starts to clamp the plug seedling pot; at the same time, the lifting module continues to drive the seedling-picking force-measuring component down, and the electric-gas coordination realizes the edge insertion and clamping of the seedling-picking force-measuring component to the plug seedling pot, to ensure the synchronization of the working process of the seedling-picking. After the seedling-picking force-measuring component completely inserts and clamps the plug seedling pot, the lifting module drives the component to clamp the plug seedling pot up to the zero height, and the component pushes the plug seedling down. In an automatic seedling-picking force-measuring experiment, two pull-pressure sensors were used to quantitatively detect the interaction between the seedling-picking force-measuring component and the plug seedling pot in real time, and then the force values of the disengaging force (F_N) and the clamping force (F_J) of the plug seedling pot during the automatic seedling-picking process were obtained.

2.1.2. Universal Design of Force-Measuring Device

The seedling-picking force-measuring component is the core working part of the force-measuring device for the automatic seedling-picking force-measuring experiment. Based on the seedling-picking end-effector developed by the research group [24], a seedling-picking end-effector with the characteristics of a centrally symmetrical clamping needle layout and a simultaneous insertion and clamping mechanism was designed, and combined with the force-measuring experimental system, it was integrated into a seedling-picking force-measuring part (Figure 2a). The seedling-picking force-measuring component is composed of a cylinder, clamping needles, turning plates, the turning plate fixed plate, seedling pushing frame, pull-pressure sensor 1, pull-pressure sensor 2, magnetic switch, and the corresponding connecting long nut, adjustable limit device, cushion block, and so on. The clamping needles and the turning plates are fixedly connected on the fixed plate of the turning plate, so that the clamping needles are collected in the guiding hole at the bottom of the seedling pushing frame, and they are distributed symmetrically along the center line of the seedling-picking force-measuring component. Through the adjustable limit device installed on the cylinder, the working stroke of the cylinder is adjusted to change the shrinkage of the clamping needle, in order to adapt to the different insertion depths of the seedling-picking force-measuring component to the plug seedling pot.

The clamping needles directly act on the plug seedling pot during the automatic seedling-picking process. When the seedling-picking force-measuring component reaches the seedling-picking point, the cylinder piston rod shrinks and drives the seedling pushing frame to lift. The clamping needles are retracted under the action of the guiding hole at the bottom of the seedling pushing frame, and the plug seedling pot is inserted and clamped at the same time with the cooperation of the lifting module. When the seedling-picking force-measuring component clamps the plug seedling pot to rise to the zero height, the cylinder piston rod extends, driving the seedling pushing frame to drop and push the plug seedling down.

The turning plate fixed plate is a special-shaped structure (Figure 3). By adjusting the installation position and number of turning plates on the turning plate fixed plate, the turning plate installation of the seedling-picking force-measuring component (2/3/4 needles) can be achieved with different numbers of central symmetrical clamping needles. At the same time, the designed seedling pushing frame has two structural forms: four-finger and three-finger. Among them, the two-needle seedling-picking force-measuring component (Figure 2a) and the four-needle seedling-picking force-measuring component (Figure 2c) can adopt the four-finger seedling pushing frame. In contrast, the three-needle seedling-picking force-measuring component (Figure 2b) needs to adopt the three-finger seedling pushing frame.

The clamping angle of plug seedling pots is critically determined by the distance between the installation point of the turning plates on the fixed plate and its centerline. To accommodate varying plug seedling specifications, three distinct installation positions (designated as Point I, Point II, and Point III) were systematically configured on the turning plate fixed plate. This modular design enables the precise adjustment of the clamping angle by relocating the turning plate installation points, thereby ensuring optimal adaptability across diverse plug seedling dimensions.

2.1.3. The Structural Parameter Model of Seedling-Picking Force-Measuring Component

The structural parameters of the seedling-picking force-measuring component in the force-measuring device should be designed based on the hole size of the seedling plugs used in the experiment [25]. In this paper, the hole sizes of common 50-hole, 72-hole and 128-hole seeding plugs were measured, respectively (

Table 1. The hole size of common seedling plugs.

Seeding Plug Specification	Hole Upper Part Opening Size/mm	Hole Lower Part Opening Size/mm	Hole Height Size/mm	Hole Cone Angle/°
50-hole	50 × 50	24 × 24	48	15.15
72-hole	40 × 40	22 × 22	42	12.09
128-hole	32 × 32	14 × 14	44	11.56

).

The mathematical model of the movement of the seedling-picking force-measuring component is constructed (Figure 4), where L₁ is the installation point of the turning plate fixed plate of the turning plate, L₂ is the length of the clamping needle, L₃ is the distance between the guide hole at the bottom of the seedling pushing frame and the center line of the seedling-picking force-measuring component, L₄ and L₄′ are the distance between the tip of the clamping needle and the guide hole at the bottom of the seedling pushing frame before the clamping needle is inserted into the plug seedling pot and when the plug seedling pot is completely clamped, respectively, L₅ is the deflection length of the turning plate, and H and H′ are the vertical distance between the guide hole at the bottom of the seedling pushing frame and the turning plate fixed plate before the clamping needle is inserted into the plug seedling pot and when the plug seedling pot is completely clamped. α and α′ are the insertion angle and clamping angle of the clamping needles for the plug seedling pot, γ is the deflection angle of the turning plate, H_e is the depth of the clamping needle inserted into the plug seedling pot, and β is the hole cone angle.

According to the measured hole size of the seeding plugs, the structural parameters of the seedling-picking force-measuring component were designed for the three specifications of the plug seedlings. The basis is as follows: when the clamping needles are completely inserted and clamped the plug seedling pot, the corresponding clamping angle α′ should not be less than half of the hole cone angle β [26,27]; after the cylinder piston rod shrinks, H′ can still be greater than the plug seedling height of common vegetable plug seedlings; when the guide hole at the bottom of the seedling pushing frame is 5 mm away from the upper surface of the plug seedling pot, the clamping needle tip just touches the upper surface of the plug seedling pot. In order to ensure that the seedling-picking force-measuring component can be compatible with three specifications of the plug seedlings at the same time, based on the 128-hole plug seedling with the smallest hole, when the clamping needles are inserted into the plug seedling pot, the gap with the inner wall of the hole is less than 2~3 mm, which is beneficial to seedling picking [28]. At the same time, a certain angle is set on the turning plates to minimize the size of the seedling-picking force-measuring component. The calculation formula of related structural parameters is as follows.

$$L_3+\left(L_2-L_4\right)sin\alpha +L_{5}sin\left[\alpha -\left(180°-\gamma \right)\right]=L_1$$

(1)

$$L_3+(L_2-L_4^{\prime })sin{\alpha }^{\prime }+L_{5}sin\left[{\alpha }^{\prime }-\left(180°-\gamma \right)\right]=L_1$$

(2)

$$L_4^{\prime }cos{\alpha }^{\prime }-L_{4}cos\alpha =H_e$$

(3)

$$Htan\alpha =L_1+L_{5}cos\left[\alpha -\left(180°-\gamma \right)\right]tan\alpha -L_{5}sin\left[\alpha -\left(180°-\gamma \right)\right]-L_3$$

(4)

$$H^{\prime }tan{\alpha }^{\prime }=L_1+L_{5}cos\left[{\alpha }^{\prime }-\left(180°-\gamma \right)\right]tan{\alpha }^{\prime }-L_{5}sin\left[{\alpha }^{\prime }-\left(180°-\gamma \right)\right]-L_3$$

(5)

$$L_3-L_{4}sin\alpha =14$$

(6)

$$L_{4}cos\alpha =5$$

(7)

Based on the above analysis, H = 180 mm, L₅ = 7.5 mm, and γ = 6 °. The known parameters are brought into the above formula to obtain the relevant parameters of the seedling-picking force-measuring component (

Table 2. Structural parameters of the seedling-picking force-measuring component of plug seedlings of different specifications.

Seedling Plug Specification	He/mm	α′/°	α/°	L1/mm	L4/mm	L4′/mm	H′/mm
50-hole	42.5	15.5	11.71	51	5.11	49.29	135.0
72-hole	36	12.5	10.05	47.5	5.08	41.99	142.5
128-hole	38	12.0	9.41	44	5.07	43.96	140.5

), where L₂ is 190 mm and L₃ is 15 mm.

The diameter of the clamping needles is too large, which will increase the damage degree of the plug seedling pot, and too small will lead to a decrease in the success rate of seedling picking. To ensure the success rate of seedling picking, the damage degree of the plug seedling pot should be minimized. The diameter of the clamping needle is optimal at 2 mm [29]. At the same time, to prevent deformation of the clamping needle, a round bar of 65 Mn is selected for making the clamping needles, which can effectively improve their stiffness.

2.2. Dynamic Analysis of Seedling-Picking Motion

2.2.1. Control of Seedling-Picking Motion Parameters

In the process of automatic seedling picking by the force-measuring device, its motion is realized by the lifting module and the cylinder. In order to adapt to the required insertion depth of plug seedlings of different specifications, the maximum stroke of the cylinder piston rod should be 50 mm. According to the preliminary research of the research group [30], selecting the AIRTAC MD series cylinder with an inner diameter of 16 mm at a working pressure of 0.3 MPa can effectively ensure the clamping action of the plug seedling pot. Under the premise of a working pressure of 0.3 MPa, the average speed is calculated to be about 280 mm/s according to the actual measurement of the time taken by the cylinder piston rod to act 100 times.

As a component that drives the lifting and shifting of the seedling-picking force-measuring component, the lifting module should act synchronously with the cylinder of the seedling-picking force-measuring component to ensure the synchronization of the process of inserting and clamping the plug seedling pot. Therefore, taking the average working speed of the cylinder as a reference, the GBF-100 ball screw module of Wuxi Ebert transmission is selected. The lead of the screw is 20 mm, the effective stroke of the module is 200 mm, the displacement accuracy is ±0.04 mm, the maximum load in the vertical direction is 10 kg, and the running speed is up to 400 mm/s.

The motion control system was meticulously designed based on the developed force-measuring device (Figure 5). The pneumatic circuit switching mechanism incorporates a sophisticated architecture combining magnetic switch state detection with solenoid valve on–off control for precise operation. A high-performance servo motor drive system was implemented to actuate the lifting module, complemented by a module proximity switch for accurate determination of the lifting module’s home position. The control architecture features a PLC controller (XD5-24T4-C) supporting high-speed USB programming, coupled with an industrial-grade touch screen (TG865-MT/(ET)). Key electromechanical components include the following:

• Servo motor—MF3H-60CS (CM) 30BZ1-504;
• Servo driver—DF3E-0410Z;
• Proximity switch—Omron TL-W3MC1;
• Solenoid valve—AIRTAC 4V210-08B (two-position five-way);
• Cylinder magnetic switch—AIRTAC CMSG020.

This integrated system configuration ensures precise force measurement and reliable control performance.

2.2.2. Dynamic Coupling Simulation of Seedling-Picking Motion via EDEM-RecurDyn

Based on clarifying the structural parameters and motion parameters of the force-measuring device, the water spinach plug seedlings were set as the experimental object, and the automatic seedling-picking coupling simulation experiment was carried out with four clamping needles for the 50-hole plug seedlings, which provided a reference for verifying the applicability of the force-measuring device.

After using SolidWorks (https://www.solidworks.com/) to establish a four-needle seedling-picking force-measuring component model for 50-hole plug seedlings, assembly and interference checks were performed. We then imported the virtual prototype model of the seedling-picking force-measuring component into RecurDyn (Figure 6a). According to the classification of the motion pairs in RecurDyn and the actual motion of the seedling-picking force-measuring component, the corresponding motion pairs were abstracted, and the rotational and moving pairs between the components were defined. At the same time, an extended surface-to-surface contact was added between each clamping needle and the corresponding guiding hole at the bottom of the seedling pushing frame. Drives were added to the moving pair between the cylinder block and the ground, and the moving pair between the cylinder piston rod and the cylinder block, respectively, to simulate the longitudinal motion of the seedling-picking force-measuring component and the telescopic motion of the cylinder piston rod.

The substrate ratio of the water spinach plug seedling pot is mainly composed of peat, perlite and vermiculite in a ratio of 3:1:1. According to previous literature [24], the radius of the block model, the columnar model and the nuclear model is randomly distributed according to 0.2~0.5 mm, the radius of the spherical model is randomly distributed according to 1.5~3 mm, and the radius of the flake model is randomly distributed according to 0.4~1 mm. The seedling pot particle model is established in a way that ensures each particle has an equal number, and a total of 54,000 particles are generated. The Hertz–Mindlin with Bonding contact model is used to simulate the cohesive force between particles and the discrete problems such as cracking and breakage of the plug seeding pot under the action of external force [30]; at the same time, the Hertz–Mindlin with JKR model is used to describe the adhesion relationship between the matrix particles and the seeding plug [31].

The 50-hole seeding plug model was established using SolidWorks, and it was imported into EDEM to correlate with the set material parameters of the seeding plug. A particle factory was established on the upper surface of the hole model to generate particles, and a seedling pot particle model of a 50-hole plug seedling pot was established. The material of the clamping needle was 65 Mn, and the material of the seeding plug was polystyrene. We consulted the relevant material performance parameters [24,32,33,34] and completed the setting of material properties in EDEM as shown in

Table 3. EDEM material properties’ settings.

Item	Attribute	Value/Unit
Particle	Poisson’s ratio	0.364
	Solids density	425 kg/m3
	Shear modulus	1.6 × 106 Pa
Seeding plug	Poisson’s ratio	0.38
	Solids density	1400 kg/m3
	Shear modulus	8.66 × 108 Pa
Clamping needle	Poisson’s ratio	0.288
	Solids density	7810 kg/m3
	Shear modulus	8.0 × 1010 Pa
Particle–Particle	Coefficient of restitution	0.12
	Coefficient of static friction	0.65
	Coefficient of rolling friction	0.345
	Normal stiffness per unit area	1 × 108
	Shear stiffness per unit area	5 × 107
	Critical normal stress	3 × 104 Pa
	Critical shear stress	1.5 × 104 Pa
	Bonded disk radius	0.6 mm
Particle–Seedling plug	Coefficient of restitution	0.4
	Coefficient of static friction	0.2
	Coefficient of rolling friction	0.002
	Surface energy	1.513 J/m2
Particle–Clamping needle	Coefficient of restitution	0.6
	Coefficient of static friction	0.397
	Coefficient of rolling friction	0.261

.

Based on the above content, the EDEM-RecurDyn coupling simulation experiment was carried out on the seedling-picking process of the clamping needles (Figure 6b). In the simulation process, the force of the clamping needles is applied to the plug seedling pot particle model in EDEM, and the data of the reaction force of the plug seedling pot particle model to the clamping needles are fed back to RecurDyn.

2.3. Force-Measuring Experimental Verification

2.3.1. Design of Experiment

The experiment was carried out in the Key Laboratory of Modern Agricultural Equipment and Technology, Ministry of Education, Jiangsu University from 27 to 29 April 2025. In this experiment, the water spinach plug seedlings (50-hole, 72-hole, 128-hole) cultivated in the greenhouse were selected as the experimental objects. The seedling age was 35 days, the matrix compaction degree was 1:1.2, and the seedling height was 100~120 mm. The seedling roots can tightly twine the matrix particles. When the plug seedlings are pulled out from the seeding plug, there will be no scattered lumps. Before the experiment, the plug seedlings were given water, so that the water content of the plug seedling pot was maintained at a moderate level of 60~65%, which was suitable for the automatic seedling picking operation [35].

In order to verify the functionality, applicability and versatility of the designed force-measuring device, different specifications of plug seedlings (including 50-hole, 72-hole and 128-hole) and different numbers of clamping needles (including two-needle, three-needle and four-needle) were used as experimental factors, and the automatic seedling-picking force-measuring was carried out through the force-measuring device (Figure 7). During the experiment, the whole plug seedlings to be tested were cut in groups and placed on the seeding plug positioning plate, and 30 holes were tested in each group. Before the experiment, the plug seedlings were manually aligned. The experimental process was automatically executed; the seedling-picking force-measuring component dropped vertically from the top of the plug seedling to be tested, and the automatic seedling picking operation was performed when the plug seedling was reached. After the completion, the plug seedling was lifted vertically to the initial height to release the plug seedling. According to the size of three different specifications of plug seedlings, the position of the seedling-picking point and the insertion depth of the clamping for the lug seedling pot were set, respectively. We connected the data collector, set the sampling frequency to 200 Hz, and collected and recorded the corresponding mechanical experimental signals.

Before the experiment, the standard values (such as pull, pressure, etc.) generated by the two pull-pressure sensors used were used as metrics to obtain the corresponding outputs and calibrate them (

Table 4. The main performance parameters of the pull-pressure sensors.

Sensor Name	Technical Parameter	Parameter Calibration
pull-pressure sensor-1	Range: 10 kg; Composite precision: ±0.05%; sensitivity: 1.12 ± 0.05 mV/V	F1 = 1.9996X1 − 0.0008 R2 = 0.999
pull-pressure sensor-2	Range: 3 kg; Composite precision: ±0.05%; sensitivity: 0.90 ± 0.04 mV/V	F2 = 0.5999X2 + 0.0038 R2 = 1.000

Note: X₁ and X₂ are the voltage signals measured by the corresponding sensors, V, respectively.

).

Before the experiment, an electronic balance was used to weigh the average mass M₁ of the tested plug seedlings before the experiment. After the experiment was completed, the average mass M₂ of the tested plug seedlings after being clamped was weighed, and then the integrity of the plug seedling pot I_R after automatic seedling-picking force-measuring experiment was calculated:

$$I_R={\displaystyle \frac{M_2}{M_1}}\times 100\%$$

(8)

After the experiment, Microsoft Excel software was used to process the data measured by the relevant pull-pressure sensors. Then, based on the measured data, the relevant mechanical curves of the disengaging force F_N and the clamping force F_J of the plug seedling pot were obtained, and the data of the key points on the relevant mechanical curves were statistically analyzed.

2.3.2. The Measured Mechanical Curve Transformation

The force values measured by the pull-pressure sensor-1 and the pull-pressure sensor-2 arranged on the seedling-picking force-measuring component are the interaction forces between the structures of the seedling-picking force-measuring component in the process of automatic seedling picking, which need to be transformed into the relevant forces of the plug seedling pot in the process through the corresponding mechanical analysis.

The relevant mechanical analysis was carried out on the disengaging force (F_N) of the plug seedling pot when the clamping needles clamped the plug seedling pot out of the seeding plug in the force-measuring device (Figure 8a). F₁ is the force of the pull-pressure sensor-1 when the seedling-picking force-measuring component clamps the plug seedling pot out of the seeding plug. F₀ is the resultant force of the pre-tightening force of the pull-pressure sensor-1, and G_S is the gravity of the plug seedling itself. The relevant mechanical transformation formula is as follow:

$$F_1=F_N+F_0+G_s$$

(9)

It can be seen that the plug seedling pot is separated from the seeding plug to overcome its internal adhesion, that is, the disengaging force of the plug seedling pot is:

$$F_N=F_1-F_0-G_s$$

(10)

The clamping force (F_J) applied to the plug seedling pot and the extrusion force (F_J′) of the plug seedling pot received during the clamping of the plug seedling pot interact with each other. The extrusion force (F_J′) takes the form of triangular distribution load. Therefore, it is necessary to equivalently transform the triangular distribution load of the extrusion force to one point. The action point of the equivalent transformed force is one-third of the maximum load concentration q, that is, the clamping force is concentrated at l from the clamping needle tip (Figure 8b).

The relevant mechanical analysis is carried out for the clamping force (F_J) applied to the plug seedling pot in the force-measuring device (Figure 8c). F₂ is the pull measured by the pull-pressure sensor-2 when the seedling-picking force-measuring component is taken, F₀′ is the pre-tightening force of the pull-pressure sensor-2 installation, n is the number of clamping needles of the seedling-picking force-measuring component (n = 2/3/4), and α′ is the angle between the clamping needle and the vertical direction when the seedling-picking force-measuring component is taken to clamp the plug seedling pot. F_S and F_S′ are interactive forces, and F_S is the extrusion force for the guide hole at the bottom of the seedling pushing frame. F_S′ is the binding force of the guide hole at the bottom of the seedling pushing frame on the clamping needle. The relevant mechanical transformation formula is as follows:

$$F_2=F_0^{\prime }+n{F}_{S}sin{\alpha }^{\prime }$$

(11)

$$F_S =F_S^{\prime }$$

(12)

$$F_S^{\prime }{l}_1=F_J^{\prime }{l}_2$$

(13)

$$F_J^{\prime } ={ F}_J$$

(14)

$$l=1/3\left(L_4^{\prime }-L_4\right)$$

(15)

$$l_1=L_2-L_4^{\prime }$$

(16)

$$l_2=L_2-l$$

(17)

It can be seen from this that the clamping force applied to the plug seedling pot by a single clamping needle is

$$F_J={\displaystyle \frac{\left(L_2-L_4\right)\left(F_2-F_0^{\prime }\right)}{nsin{\alpha }^{\prime }\left[L_2-1/3\left(L_4^{\prime }-L_4\right)\right]}}$$

(18)

3. Results

3.1. Functional Analysis of Force-Measuring Device

The process of automatic seedling picking by the designed force-measuring device is as follows: the seedling-picking force-measuring component drops to the seedling-picking point as a whole, the clamping needles insert and clamp the plug seedling pot, the seedling-picking force-measuring component takes the plug seedling out of the seedling plug and rises as a whole, and the seedling-picking force-measuring component pushes the plug seedling down (Figure 9). The main motion of the designed automatic seedling-picking force-measuring device is borne by the lifting module driven by the servo motor. Due to the accuracy of the servo system, the force-measuring device can complete the seedling-picking action smoothly and coherently.

Figure 10 shows the relevant mechanical curves of the automatic seedling picking experiment for 50-hole plug seedlings. The orange curve represents the relevant mechanical curve F₁ measured by the pull-pressure sensor 1, the green curve is the relevant mechanical curve F₂ measured by the pull-pressure sensor 2, the blue curve represents the F_N correlation curve of the disengaging force transformed by the F₁ (Formula (10)), and the purple curve represents the F_J correlation curve of the clamping force transformed by the F₂ (Formula (18)).

Stage I involves the seedling-picking force-measuring component dropping at a constant speed (150 mm/s) under the drive of the lifting module, and reaching the seedling-picking point at 0.8 s. At this time, the clamping needle tip is just in contact with the upper surface of the plug seedling pot. The relevant mechanical curves measured by the pull-pressure sensor 1 (F₁ = F₀) and the relevant mechanical curves measured by the pull-pressure sensor 2 (F₂ = F₀′) remain approximately constant, and there is no significant change.

Stage II combines the lifting module (operating speed of 280 mm/s) and the cylinder (with a piston rod expansion pressure of 0.3 MPa), enabling the insertion and clamping of the clamping needles into the plug seedling pot. At 0.8 s, the clamping needles began to insert and clamp the plug seedling pot. The relevant mechanical curves measured by the pull-pressure sensor 1 and the relevant mechanical curves measured by the pull-pressure sensor 2 jumped synchronously, and reached the peak point at about 0.96 s. It can be noted that the clamping force (F_J) of the single clamping needles for the plug seedling pot and the oblique insertion force (F_C) during the insertion and clamping of the clamping needles increase linearly with the increase in the insertion depth of the plug seedling pot until the peak value of the clamping force (F_Jmax) and the peak value of the oblique insertion force (F_Cmax) are reached when the plug seedling pot is completely inserted and clamped. At this time, the cylinder piston rod shrinks in place, and the lifting module stops running.

Stage III comprises a short stay after the clamping needles are completely inserted and clamped into the plug seedling pot. Due to the impact of the cylinder piston rod, the relevant mechanical curves measured by the pull-pressure sensor 2 show large fluctuations. For this reason, the force-measuring device is set to be temporarily stationary to prevent the subsequent measured mechanical curves from being affected by the impact, resulting in a distortion of the experimental results. At the same time, within the last approximately 0.03 s of this stage, there is a decreasing trend in the clamping force F_J correlation curve, which may be due to the fact that as the force-measuring device gradually returns to stability, the plug seedling is still in the seeding plug, and the clamping force applied by the clamping needles on the plug seedling tends to relax, maintaining a brief equilibrium with the plug seedlings.

Stage IV is when the seedling-picking force-measuring component takes the plug seedling out of the seeding plug and rises as a whole. In the first 0.04 s of this stage, the secondary lifting of the clamping force (F_J) correlation curve enables the plug seedling pot to receive sufficient clamping force to overcome its adhesion to the inner wall of the seeding plug. Due to the instantaneous relaxation of the matrix caused by the impact of the cylinder piston rod in place on the plug seedling pot in stage II, the correlation curve of the clamping force (F_J) in stage IV shows a certain drop compared with the peak value of the clamping force (F_Jmax). At about 1.14 s, the disengaging force (F_N) correlation curve reached the peak value of the disengaging force (F_Nmax), which corresponded to the critical point of root–hole wall adhesion and fracture. At this time, the plug seedling pot was completely separated from the inner wall of the seeding plug. After that, the relevant mechanical curves measured by the pull-pressure sensor 1 were stable in the combined force of the pre-tightening force and the gravity of the plug seedling (F₁ = F₀ + G_S), and the clamping force (F_J) correlation curve remained stable synchronously.

Stage V involved taking the seedling-picking force-measuring component to push the plug seedling down. At this time, the seedling-picking force-measuring component was separated from the plug seedling, resulting in the relevant mechanical curves measured by the pull-pressure sensor 1 (F₁ = F₀) and the relevant mechanical curves measured by the pull-pressure sensor 2 (F₂ = F₀′) plummeting to the pre-tightening force (F₁ = F₀, F₂ = F₀′). The disengaging force (F_N) and the clamping force (F_J) returned to zero, and a mechanical cycle was completed.

3.2. Applicability Analysis of Force-Measuring Device

Under the set motion conditions, the four clamping needles were applied in the 50-hole plug seedling pot particle model to carry out the coupling simulation experiment of the clamping seedling, and the relevant mechanical curve (Figure 11) was obtained. In order to reduce the amount of calculation, the coupling simulation experiment of seedling picking in this study only analyzed the stages II to IV in Figure 10. In these three stages, the clamping needles were in direct contact with the plug seedling pot. The orange curve represents the disengaging force (F_Nt) correlation curve of the plug seedling pot particle model in the coupling simulation experiment, and the green curve represents the clamping force (F_Jt) correlation curve applied to the seedling pot particle model by a single clamping needle in the coupling simulation experiment.

The plug seedling pot particle model was only subjected to the disengaging force (F_Nt) of the clamping needles lifted upward to take it out of the hole model in stage III. The disengaging force (F_Nt) of the plug seedling pot particle model began to increase from the time of 0.3 s, and it began to decrease to zero after reaching the peak value (F_Ntmax) at about 0.34 s. At 0.34 s, the plug seedling pot particle model was completely separated from the hole model. For the coupling simulation experiment, the disengaging force (F_Nt) correlation curve of the plug seedling pot particle model is approximately consistent with the trend of the disengaging force F_N curve of the plug seedling pot in Figure 10, and the peak value of disengaging force (F_Ntmax = 3.62 N) and the appearance time (0.04 s after starting to leave the hole model) are relatively close.

For the correlation curve of the clamping force (F_Jt) applied to the plug seedling pot particle model by a single clamping needle in the coupling simulation experiment, in stage II, as the clamping needles were inserted into the plug seedling pot particle model, the clamping force (F_Jt) of the single clamping needle of the plug seedling pot particle model gradually increased, but this was not an absolute linear change trend. The main reason is that the particles in the plug seedling pot particle model were randomly distributed, and there was still a certain gap inside the compaction. However, the particle gap between the plug seedling pot matrix used in the actual experiment was small, resulting in a certain error between the reaction force of the plug seedling pot particle model and the actual situation in the process of inserting the clamping needles into the seedling pot particle model, but the overall trend was close to that shown in Figure 11, and the size of the peak value of the clamping force (F_Jtmax = 6.54 N) and the occurrence time (0.16 s after the clamping needles are inserted into the seedling pot particle model) also showed small errors. In stage III, the clamping force (F_Jt) applied to the plug seedling pot particles model began to fall back briefly and tended to be stable. In Figure 10, due to the impact of the cylinder piston rod, the mechanical curve measured by the pull-pressure sensor fluctuated greatly, but the overall trend was similar. In the initial stage of stage IV, the correlation curve of clamping force (F_Jt) was also different from that in Figure 10. The reason is that the stop time set by the force-measuring device was short, and the impact of the cylinder piston rod was not completely eliminated. With the clamping needles clamping the plug seedling pot particle model out of the hole model, the clamping force (F_Jt) began to stabilize, but because the clamping needles continuously applied the clamping force to the plug seedling pot particle model, the particles would produce less shedding, which made the clamping force (F_Jt) show a decreasing trend.

In summary, the disengaging force F_N and clamping force F_J curves of the plug seedling pot measured by the force-measuring device in Figure 10 are similar to the disengaging force (F_Nt) and clamping force (F_Jt) curves of the plug seedling pot particle model obtained by the coupling simulation experiment in Figure 11, which proves that the force-measuring device is suitable for exploring the dynamic coupling process between the seedling-picking device and the plug seedling pot during the automatic seedling-picking process.

3.3. Universality Analysis of Force-Measuring Device

The correlation of the peak value of the disengaging force (F_Nmax) of the plug seedlings in the mechanical experiment were analyzed (Figure 12a). Taking four clamping needles as an example, the range of the F_Nmax of the 50-hole plug seedlings was 3.39~3.71 N; the range of the F_Nmax of 72-hole plug seedling was 1.87~2.18 N; for the 128-hole plug seedlings, the range of the F_Nmax of the seedling pot was 0.97~1.19 N. As shown in

Table 5. Variance analysis of related mechanical properties of plug seedlings.

Force	Source	Sum of Squares	Freedom	Mean Square	F Value	p Value	Significance
FNmax	Seedling plug specification	310.107	2	155.053	18,350.197	0.000	**
	The number of clamping needles	4.199	2	2.100	248.476	0.000	**
FJmax	Seedling plug specification	2069.470	2	1034.735	24,272.779	0.000	**
	The number of clamping needles	862.124	2	431.062	10,111.837	0.000	**

Note: ** means significant level at p < 0.01.

, the effects of the seedling plug specification on F_Nmax were extremely significant (p < 0.01).

When the number of clamping needles is the same, with the decrease in the size of the plug seedling pot, the peak value of the disengaging force (F_Nmax) of the plug seedling pot decreases when the clamping needles clamp the plug seedling out of the seeding plug. The main reason is that with the decrease in the hole volume, the contact area between the pot seedling substrate and the inner wall of the hole decreases. At the same time, with the decrease in the size of the plug seedling pot, the clamping volume of the plug seedling pot is larger in the designed seedling-picking force-measuring component. Therefore, in order to make the clamping needles successfully clamp the plug seedling out of the seeding plug, in the process of inserting and clamping the plug seedling pot, it should be ensured that the clamping needles grasp most of the root–soil composite structure of the plug seedling pot, forcing the internal matrix of the plug seedling pot to gradually loosen and then driving the seeding-picking force-measuring component to the plug seedling to lift up and separate from the seeding plug, which has the effect of stabilizing the clamping plug seeding pot.

Taking the 50-hole plug seedlings as an example, the range of the F_Nmax of the plug seedlings was 3.77~4.09 N when the two clamping needles were used. The range of the F_Nmax of the plug seedlings was 3.62~3.94 N when three clamping needles were used. The range of the F_Nmax of the plug seedlings was 3.39~3.71 N when four clamping needles were used. As shown in Table 5, the number of clamping needles also has a significant impact on F_Nmax (p < 0.01).

For plug seedlings of the same specification, the F_Nmax gradually decreased with an increase in the number of clamping needles. Due to the inclined clamping of the clamping needles, the plug seedling pot was forced to deform upward and loosen, which is equivalent to applying the prying action from the inside of the plug seedling pot [36]. Therefore, with the increase in the number of clamping needles, a stronger encapsulation of the plug seedling pot can be induced, so that the clamping forces of the clamping needles are more uniform. At the same time, the internal gap caused by the clamping needles’ insertion into the plug seedling pot increases, forcing the internal matrix of the plug seedling pot to loosen, and weakening the adhesion between the plug seedling pot and the hole to a certain extent.

The peak value of the clamping force (F_Jmax) of the plug seedlings in the mechanical experiment were analyzed (Figure 12b). Taking four clamping needles as an example, the range of the F_Jmax of a single clamping needle in the 50-hole plug seedling pots was 6.28~7.05 N; the range of the F_Jmax of the single clamping needle in the 72-hole plug seedling pots was 3.30~3.98 N; the range of the F_Jmax of the single clamping needle in the 128-hole plug seedling pots was 1.76~2.35 N. As shown in Table 5, the effect of the seedling plug specifications on F_Jmax is extremely significant (p < 0.01).

When the number of clamping needles is the same, as the size of the plug seedling pot decreases, the peak value of the clamping force (F_Jmax) applied to the seedling pot by the single clamping needle decreases, and there is a certain rule affecting the change of the F_Nmax. From the above analysis, it can be seen that with the decrease in the size of the plug seedling pot, the adhesion force between the plug seedling pot and the seedling plug that the plug seedling pot needs to overcome is reduced, and the clamping force (F_J) of the plug seedling pot is mainly used to overcome the adhesion force between the plug seedling pot and the seedling plug.

Taking 50-hole plug seedlings as an example, the range of the F_Jmax of a single clamping needle in the plug seedling pot was 12.99~13.97 N when the two clamping needles were used; the range of the F_Jmax for a single clamping needle in the plug seedling pot was 8.54~9.41 N when the three clamping needles were used; the range of the F_Jmax of the single clamping needle in the plug seedling pot was 6.28~7.05 N when the four clamping needles were clamped. As shown in Table 5, the number of clamping needles also has a significant impact on F_Jmax (p < 0.01).

For plug seedlings with the same specification, with the increase in the number of clamping needles, the peak value of the clamping force (F_Jmax) applied to the plug seedling pot by a single clamping needle gradually increases linearly, and the proportional relationship is close to the proportional relationship of the number of clamping needles. Therefore, it can be concluded that for the plug seedlings of the same specification, the number of different clamping needles causes little difference in the resultant force of the clamping force applied to the plug seedling pot, and it is synchronized with the F_Jmax of the plug seedling pot in Figure 12a. The clamping force of the clamping needle on the plug seedling pot can be changed by adjusting the number of clamping needles. Excessive clamping force will break the plug seedling pot. Therefore, in order to ensure a better seedling-picking effect, the resultant force of the clamping force applied to the plug seedling pot should be within a reasonable range.

We analyzed the integrity of the plug seedling pot with different specifications of plug seedlings and different numbers of clamping needles, as shown in

Table 6. Integrity analysis of plug seedling pot.

Seedling Plug Specification	The Number of Clamping Needles	M1/g	M2/g	IR/%
50-hole	2	34.25	33.28	97.17
	3	33.93	32.86	96.85
	4	35.62	34.34	96.41
72-hole	2	20.97	20.47	97.62
	3	22.43	21.77	97.06
	4	21.62	20.91	96.72
128-hole	2	13.25	13.06	98.56
	3	13.12	12.88	98.17
	4	12.47	12.16	97.51

. The success rate of using the force-measuring device to remove the plug seedlings from the seedling plug clamp during the experiment was 100%, and the integrity of the plug seedling pot (I_R) was above 96% under the experimental conditions. This proves that the force-measuring device designed in this study can complete the automatic seedling-picking force-measuring experiment while ensuring a low rate of damage to the plug seedling pot.

4. Discussion

A force-measuring device was proposed in this study that enables the quantitative investigation of the dynamic interaction mechanisms between the seedling-picking device and plug seedlings during automated transplantation processes. The novel force-measuring device can carry out automatic seedling-picking force-measuring experimental research for different specifications of plug seedlings and different numbers of clamping needles. By providing precise force measurement capabilities to verify its functionality, applicability and versatility, the device can become a standardized experimental platform, thereby contributing to the optimization of transplanting machine performance.

Regarding the functionality of the force-measuring device, this study successfully developed a force-measuring device founded on the centrally symmetrical clamping needle layout and a simultaneous insertion and clamping mechanism. Through modular design (supporting 2/3/4 needles switching) and parameter stepless adjustment (adapting to 50-hole/72-hole/128-hole), the dynamic response curves of the clamping force and the disengaging force of the plug seedlings during the automatic seedling picking process were accurately obtained. Compared with the built-in pot seedling clamping force sensor of Jin et al. [19], the influence of the sensor structure and installation method on the interference of the standard pick-and-throw action of the seedling-picking end-effector is effectively solved. However, the required clamping needle size is larger, which increases the likelihood of causing damage to the plug seedling pot matrix during the actual seedling picking process. Their study only detected the clamping force of the plug seedling pot. In this study, the time series coupling relationship between the clamping force F_J and the disengaging force F_N of the plug seedling pot during the seedling-picking operation was detected synchronously by the dual sensor system. In the experiment, a specific relationship was identified between the change in clamping force and the disengaging force of the plug seedling pot during the automatic seedling-picking process, consistent with the conclusion drawn by Han et al. [21] using a designed force-measuring device.

To verify the applicability of the force-measuring device, water spinach plug seedlings were used as the experimental object in this research. Through the combination of EDEM-RecurDyn coupling simulation experiment and mechanical verification experiment, the coupling simulation experiment can obtain the dynamic coupling relationship between the disengaging force (F_Nt) and the clamping force (F_Jt) of the seedling pot of the plug seedling pot particle model during the seedling-picking process. The experimental signal fluctuation resulting from the cylinder piston rod’s impact is reasonably predicted in the mechanical verification experiment’s results. The experimental results show that the disengaging force F_N curve and the clamping force (F_J) curve measured by the force-measuring device in Figure 10 are highly consistent with the disengaging force (F_Nt) curve and the clamping force (F_Jt) curve of the plug seedling pot particle model obtained by the coupling simulation experiment in Figure 11. The force-measuring device designed in this study can explore the dynamic coupling process between the seedling-picking device and the plug seedling. Jiang et al. [17] tested the interaction force between the designed four-needle seedling-picking end-effector and the plug seedling only by arranging the pull-pressure sensor. Tian et al. [21] qualitatively described this phenomenon only through EDEM simulation, and both lacked a comparative analysis of theoretical data and actual data.

Following the verification of the designed force-measuring device’s functionality and applicability, this study conducted a universal verification experiment on the device using plug seedlings of different specifications and varying numbers of clamping needles, and analyzed the experimental results. For the same number of clamping needles, with the decrease in the size of the plug seedling pot, the peak value of the disengaging force (F_Nmax) required for the plug seedling pot to separate from the seeding plug due to the change of the quality of the plug seedling pot and the contact area with the seeding plug was also correspondingly reduced, which was mainly due to the decrease in the size of the plug seedling pot, which led to a decrease in the contact area with the inner wall of the seeding plug hole and the overall quality of the plug seedling, thus weakening the adhesion between the substrate and the seeding plug wall. The peak value of the clamping force (F_Jmax) applied to the plug seedling pot by a single clamping needle also decreased exponentially. This phenomenon is closely related to the change trend of the disengaging force, indicating that in the process of automatic seedling picking, the clamping force is mainly used to overcome the adhesion resistance between the plug seedling pot and the seeding plug. There is a significant synchronous variation between the disengaging force and the clamping force, which reflects the intrinsic correlation between the two in the mechanical mechanism. For plug seedlings of the same specifications, increasing the number of clamping needles results in a decreasing trend in the peak disengaging force (F_Nmax) required to clamp the plug seedling away from the seeding plug. The main reason is that the simultaneous insertion and clamping of the clamping needles causes the pre-split zone inside the plug seedling pot to loosen during the seedling-picking process. The more clamping needles used, the more obvious the effect; at the same time, the peak value of the clamping force (F_Jmax) applied by a single clamping needle for the plug seedling pot is also approximately proportionally reduced. The above is due to the increase in the number of clamping needles, which leads to an increase in the number of force points and a more uniform distribution of the plug seedling pot. Under the premise that the total clamping force remains relatively stable in order to meet the needs of seedling picking, the load dispersed to each clamping needle is reduced accordingly. This decrease is basically consistent with the proportion of the increase in the number of clamping needles. Based on the content above, it can be concluded that the force-measuring device designed in this study can conduct automatic seedling-picking force-measuring experiments for various plug seedlings specifications and different numbers of clamping needles, thereby meeting general requirements.

A force-measuring device was proposed and validated in its functionality, applicability, and universality. However, several limitations still persisted, as follows: the verification process for crops was relatively simplistic, the test parameters were somewhat fixed, and the number of coupled verification was limited. These factors constrain the device’s overall functionality, applicability, and universality to some extent, indicating areas for future testing and enhancement.

5. Conclusions

In this research, a universal automatic seedling-picking force-measuring device was successfully developed, which is based on the central symmetric clamp needle layout and the simultaneous insertion and clamping mechanism. The modular design enables the flexible adjustment of the number of clamping needles (2/3/4 needles), which can be adapted to the insertion depths (42.5 mm, 36 mm, 38 mm) and clamping angles (12°, 12.5°, 15.5°) of 50-hole, 72-hole, and 128-hole plug seedlings. Its functionality, applicability, and universality were verified through simulation and experimentation. The main conclusions are presented as follows:

1. A real-time monitoring system with dual pull-pressure sensors was integrated to precisely acquire the relevant mechanical curves during the automatic seedling-picking process. Through formula transformation, the dynamic response curves of the disengaging force (F_N) and the clamping force (F_J) were accurately obtained, thereby validating the functionality of the force-measuring device;

2. The results of the EDEM-RecurDyn coupling simulation experiment were highly consistent with those of the mechanical experiment. Specifically, the errors between the peak value of the disengaging force (F_Nmax) and the peak value of the clamping force (F_Jmax) were minor, and the dynamic response curves of the disengaging force (F_N) and the clamping force (F_J) were approximately identical. The above proves that the device can effectively explore the dynamic coupling process between the seedling-picking device and the plug seedling, and verifies the applicability of the force-measuring device;

3. A systematic force-measuring experiment for seedling-picking was conducted on plug seedlings of different specifications (50-hole, 72-hole, 128-hole) and with different numbers of clamping needles (2/3/4 needles). The results indicate that, regarding the characteristics of the disengaging force, the peak value of the disengaging force (F_Nmax) increased significantly as the hole size increased, and an increase in the number of clamping needles contributed to a reduction in F_Nmax; regarding the characteristics of the clamping force, the peak value of the clamping force (F_Jmax) applied to the plug seedling pot by a single clamping needle was approximately inversely proportional to the number of clamping needles, and the distribution of the clamping force on the plug seedling pot under the four-needle layout was the most uniform. These results verify the general requirements of the designed force-measuring device for conducting the automatic seedling-picking force-measuring experiment.