In modern agriculture, the mechanization of crop harvesting is critical for improving efficiency and reducing labor costs. Safflower, valued for its oil and medicinal properties, presents a unique challenge due to the delicate nature of its filaments, which are prone to damage during mechanical harvesting. Traditional harvesters often prioritize filament removal rates but result in high filament breakage, compromising product quality. To address this, we have developed a novel double-acting opposite direction cutting end effector. This end effector is designed to achieve low-damage harvesting by implementing slow-speed clamping and cutting with segmented operation, thereby minimizing filament fragmentation while ensuring high removal rates. The core innovation lies in the end effector’s ability to perform precise, controlled cuts through a dual-knife system and a multi-segment cam mechanism. This article details the design, theoretical analysis, experimental optimization, and validation of this end effector, providing a comprehensive framework for advancing safflower harvesting technology.
The primary physical characteristics of safflower must be considered in the design of any harvesting end effector. Key parameters include the capitulum diameter, neck diameter, filament spread width, and the peak cutting force required at the neck region. Based on field measurements, typical values are summarized in Table 1. These parameters directly influence the dimensional and operational specifications of the end effector components.
| Parameter | Range | Average Value |
|---|---|---|
| Capitulum Diameter (d₁) | 19.47 – 28.95 mm | 25.15 ± 2.30 mm |
| Neck Diameter (d₂) | 2.82 – 6.62 mm | 4.73 ± 0.78 mm |
| Filament Spread Width (w₁) | 41.55 – 54.31 mm | 47.85 ± 3.73 mm |
| Capitulum Height (w₂) | 17.12 – 27.25 mm | 21.58 ± 2.00 mm |
| Peak Cutting Force (Fₜ) | 70.12 – 108.23 N | 89.17 ± 6.13 N |
| Filament Moisture Content | 50.6 – 80.4 % | 61.5 % |
The overall safflower harvesting machine integrates this end effector with spatial adjustment mechanisms, a mobile platform, a negative-pressure collection system, and an electronic control unit. The end effector is mounted at the terminus of the spatial adjustment module. Upon receiving positional data of the safflower capitulum, the system aligns the end effector for harvesting. The cut filaments are then collected via the negative-pressure system. The focus of this work is the double-acting opposite direction cutting end effector itself, which is the critical component responsible for the precise separation of filaments from the capitulum.
The double-acting opposite direction cutting end effector comprises several key sub-assemblies: the double-acting cutting component, the tool feed mechanism, a conical flower-guiding机构, a housing, and a stepper motor. The operational principle involves the filaments being guided and straightened by the conical机构 under negative pressure, reducing their spread. Subsequently, the stepper motor activates, causing the double-acting cutting component to open. The tool feed mechanism then drives the cutting knives through a sequence of “acceleration, constant-speed cutting, deceleration, and stop” to sever the filaments. After cutting, the knives remain closed to prevent filament drop, allowing the collected filaments to be conveyed by the negative-pressure system. Finally, the knives retract to their initial position. This segmented operation is crucial to avoid multiple cuts and reduce damage, a core advantage of this end effector design.
The double-acting cutting component is the heart of the end effector. It consists of an upper knife, a lower knife, knife holders, linear slides, and rolling bearings. The knives converge along slides to cut the filaments. Key geometric parameters of the knives include length L₁, width L₂, thickness L₃, edge tilt angle β, and blade face inclination angle θ. The gap H between the upper and lower knives is minimized to prevent filaments from being ground and damaged. Based on standards and preliminary tests, we set L₁ = 70 mm, L₂ = 30 mm, L₃ = 2.5 mm, and H = 0.6 mm. The conical guide aperture D is designed to be slightly smaller than the minimum capitulum diameter to prevent the capitulum from entering the cutting zone, with D = 19 mm.
To understand the cutting mechanics and optimize the end effector, we developed a theoretical model for the tool-filament interaction. The cutting process involves the impact of the knife on the filament. Applying the impulse-momentum theorem:
$$ F t = m v $$
where F is the impact force (N), t is the cutting time (s), m is the filament mass (g), and v is the knife feed speed (mm/s). The filament, being elastic, deforms upon impact. The critical condition for filament fracture is:
$$ F \ge \sigma_s A $$
Here, σ_s is the ultimate tensile stress (Pa) of the filament, and A is the cutting area (mm²). The cutting area can be expressed as:
$$ A = 2 l_1 L_3 \tan\left(\frac{\theta}{2}\right) \cos\beta $$
where l₁ is the cutting depth (mm). Combining these equations yields the fracture condition:
$$ \frac{F}{A} = \frac{m v^2 \cos\beta}{2 l_1^2 L_3 \tan(\theta/2)} \ge \sigma_s $$
This indicates that filament fracture depends on the knife feed speed v, the edge tilt angle β, and the blade face inclination angle θ. To ensure cutting while minimizing damage, these parameters must be carefully balanced. For the filament to be severed, the feed speed must satisfy:
$$ v \ge \sqrt{\frac{2 \times 10^6 F_t l_1^2 L_3 \tan(\theta/2)}{A_s m \cos\beta}} $$
Using measured values (m ≈ 1.55 g, maximum l₁ ≈ d₂_max = 6.62 mm, F_t_max ≈ 108.23 N, A_s ≈ 17.57 mm²), we find v ≥ 9.69 mm/s. However, higher speeds increase impact and damage. Based on theoretical analysis and preliminary tests, an optimal feed speed range of 20–40 mm/s was identified for low breakage and effective cutting in this end effector.
Furthermore, to prevent filament slippage during the clamping cut, we analyzed the force equilibrium at the neck. For the filament not to slip out, the condition is:
$$ \beta_1 + \beta_2 \le 2\phi $$
where β₁ and β₂ are the edge tilt angles of the upper and lower knives, and φ is the friction angle between the knife and filament (measured as φ ≈ 23°). Therefore, to ensure proper clamping, we require β₁ = β₂ ≤ 23°.
From a microscopic perspective, the cutting resistance during shearing is composed of forces from the blade edge and faces. The total cutting force P required can be derived as:
$$ P = \Delta l_2 \sigma_s + mg\tan\phi + \frac{E h}{2} \left[ \tan\theta + f(\sin 2\theta + \mu \cos 2\theta) \right] $$
where Δ is the edge thickness, l₂ is the effective blade length, E is the elastic modulus of the filament (≈ 2.5 × 10⁶ Pa), h is the filament thickness, f is the friction coefficient, and μ is Poisson’s ratio (≈ 0.25). This equation shows that P increases with θ, but the trend is relatively mild for θ ≤ 30°. Thus, to reduce cutting force and filament damage, the blade face inclination angle θ should be kept below 30°. These theoretical insights guided the parameter ranges for our experimental optimization of the end effector.
The tool feed mechanism in the end effector enables segmented operation, which is vital to avoid multiple cuts during filament feeding and collection. We employed a cylindrical cam with two symmetrical grooves to drive the reciprocating motion of the knives via rolling bearings. The cam profile was designed to divide the operation into four distinct segments: cam start segment, cutting and separation segment, knife dwell segment, and knife return segment. This segmentation ensures that the knives are stationary during filament feeding and post-cut collection, preventing re-cutting and fragmentation.
The cam motion law was carefully selected to minimize shock and damage. For the cutting segment, a combination of constant velocity and cycloidal motion (sine acceleration) was used to avoid rigid impact. For the return segment, simple harmonic motion (cosine acceleration) was applied. The cam rotation angles for each segment were allocated as follows: ω₁ = π/6 (start), ω₂ = π/6 (acceleration), ω₃ = π/2 (constant-speed cutting), ω₄ = π/6 (deceleration), ω₅ = π/3 (dwell), and ω₆ = 2π/3 (return). The total cam lift u was calculated based on the need for the knives to fully cover the guide aperture during cutting:
$$ u_q \ge \frac{D(1 + \sin\beta)}{2\cos\beta} $$
This yielded u_q ≥ 18 mm. With the allocated angles, the total lift u = 24 mm, with acceleration lift u_a = 3 mm and deceleration lift u_b = 3 mm. The knife displacement s as a function of cam rotation angle ω is given by:
$$ s(\omega) = \begin{cases}
0 & (0 \le \omega < \frac{\pi}{6}) \\
3\left(\frac{6\omega – \pi}{\pi} – \frac{\sin(6\omega – \pi)}{\pi}\right) & (\frac{\pi}{6} \le \omega < \frac{\pi}{3}) \\
3 + \frac{12(3\omega – \pi)}{\pi} & (\frac{\pi}{3} \le \omega < \frac{5\pi}{6}) \\
24 – 3\left[\frac{6(\pi – \omega)}{\pi} + \frac{3\sin(6(\pi – \omega))}{\pi}\right] & (\frac{5\pi}{6} \le \omega < \pi) \\
24 & (\pi \le \omega < \frac{4\pi}{3}) \\
12\left(1 + \cos\left(\frac{3}{2}\omega – 2\pi\right)\right) & (\frac{4\pi}{3} \le \omega < 2\pi)
\end{cases} $$
The corresponding velocity and acceleration profiles are smooth, ensuring minimal impulsive loads on the filaments. This cam design is integral to the precise and gentle operation of the end effector.
To evaluate and optimize the performance of the double-acting opposite direction cutting end effector, we conducted a series of experiments. The test setup included the end effector prototype, a filament collection box, a power supply, and a stepper motor controller. Key instruments comprised an electronic balance, calipers, a tachometer, and a texture analyzer. The safflower variety used was ‘Jin Hong 8’ at full bloom stage.
We designed a three-factor, five-level quadratic orthogonal rotation combination test to investigate the effects of critical parameters on harvesting performance. The factors were: cam speed x₁ (directly related to knife feed speed v), edge tilt angle x₂, and blade face inclination angle x₃. The response indices were filament removal rate y₁ and filament broken rate y₂, defined as:
$$ y_1 = \frac{m_1}{m_1 + m_2} \times 100\% $$
$$ y_2 = \frac{m_3}{m_1} \times 100\% $$
where m₁ is the mass of harvested filaments from a capitulum, m₂ is the mass of remaining filaments, and m₃ is the mass of broken filaments among the harvested ones. The factor levels are coded in Table 2.
| Code | Cam Speed x₁ (r/min) | Edge Tilt Angle x₂ (°) | Blade Face Inclination Angle x₃ (°) |
|---|---|---|---|
| -1.682 | 11.1 | 9.3 | 11.6 |
| -1 | 16.8 | 12.0 | 15.0 |
| 0 | 25.2 | 16.0 | 20.0 |
| 1 | 33.6 | 20.0 | 25.0 |
| 1.682 | 39.3 | 22.7 | 28.4 |
The experimental matrix comprised 23 runs, including 9 center points for estimating pure error. Each test was repeated three times, and average values were taken. The results are summarized in Table 3.
| Run | X₁ | X₂ | X₃ | y₁ (%) | y₂ (%) |
|---|---|---|---|---|---|
| 1 | -1 | 1 | 1 | 87.03 | 7.02 |
| 2 | -1 | -1 | -1 | 86.05 | 7.65 |
| 3 | -1 | 1 | -1 | 86.76 | 5.46 |
| 4 | -1 | -1 | 1 | 87.79 | 6.87 |
| 5 | 1 | 1 | -1 | 89.37 | 6.37 |
| 6 | 1 | -1 | 1 | 90.09 | 7.51 |
| 7 | 1 | 1 | 1 | 89.29 | 7.74 |
| 8 | 1 | -1 | -1 | 90.05 | 7.59 |
| 9 | -1.682 | 0 | 0 | 88.54 | 5.88 |
| 10 | 1.682 | 0 | 0 | 92.33 | 7.02 |
| 11 | 0 | -1.682 | 0 | 88.56 | 7.55 |
| 12 | 0 | 1.682 | 0 | 87.28 | 7.32 |
| 13 | 0 | 0 | -1.682 | 87.24 | 7.67 |
| 14 | 0 | 0 | 1.682 | 87.76 | 8.23 |
| 15 | 0 | 0 | 0 | 91.25 | 5.38 |
| 16 | 0 | 0 | 0 | 91.44 | 5.25 |
| 17 | 0 | 0 | 0 | 91.15 | 5.02 |
| 18 | 0 | 0 | 0 | 91.31 | 4.95 |
| 19 | 0 | 0 | 0 | 91.68 | 5.76 |
| 20 | 0 | 0 | 0 | 91.26 | 5.07 |
| 21 | 0 | 0 | 0 | 91.39 | 5.04 |
| 22 | 0 | 0 | 0 | 91.71 | 5.05 |
| 23 | 0 | 0 | 0 | 91.72 | 5.26 |
Using Design-Expert software, we performed analysis of variance (ANOVA) on the data. The regression models for the response indices, after eliminating non-significant terms, are:
$$ y_1 = 91.44 + 1.28X_1 – 0.27X_2 + 0.21X_3 – 0.26X_1X_3 – 0.38X_1^2 – 1.27X_2^2 – 1.42X_3^2 $$
$$ y_2 = 5.21 + 0.30X_1 – 0.25X_2 + 0.22X_3 + 0.47X_2X_3 + 0.36X_1^2 + 0.71X_2^2 + 0.89X_3^2 $$
where X₁, X₂, X₃ are coded factors. The ANOVA indicated that for y₁, the main effects of X₁, X₂, X₃, and quadratic terms X₁², X₂², X₃² were highly significant (P < 0.01), while the interaction X₁X₃ was significant (P < 0.05). For y₂, X₁, X₂X₃, X₁², X₂², X₃² were highly significant, and X₂, X₃ were significant. The lack-of-fit tests were non-significant (P > 0.05), confirming model adequacy.
The response surface analysis revealed the influence of factors on performance. For filament removal rate y₁, higher cam speed generally increased y₁, but the effect diminished at higher speeds. The edge tilt angle and blade face inclination angle had quadratic effects, with optimal ranges around mid-levels. For filament broken rate y₂, both angles exhibited quadratic relationships, with minimum breakage occurring at moderate angles. The interaction between edge tilt angle and blade face inclination angle was significant for breakage, indicating that these parameters must be tuned together in the end effector design.
To achieve multi-objective optimization—maximizing y₁ and minimizing y₂—we used the desirability function approach in Design-Expert. The constraints were: 11.1 r/min ≤ x₁ ≤ 39.3 r/min, 9.3° ≤ x₂ ≤ 22.7°, 11.6° ≤ x₃ ≤ 28.4°. The optimal parameter combination was found to be: cam speed x₁ = 27.9 r/min, edge tilt angle x₂ = 16.1°, blade face inclination angle x₃ = 19.7°. At this point, the predicted responses are y₁ = 91.78% and y₂ = 5.32%.
We conducted field validation tests with this optimal parameter set. The results showed an average filament removal rate of 91.25% and a filament broken rate of 5.57%. The errors relative to predicted values are less than 5%, confirming the accuracy of the regression models and the effectiveness of the optimized end effector. This demonstrates that the double-acting opposite direction cutting end effector can achieve high-efficiency, low-damage harvesting of safflower filaments.
The success of this end effector lies in its integrated design principles: low-speed dual-knife clamping cutting reduces impact forces; optimized knife geometry minimizes slippage and cutting resistance; and segmented cam-driven operation prevents multiple cuts. Compared to conventional high-speed cutting or pulling mechanisms, this end effector significantly lowers filament breakage while maintaining high harvest efficiency. The theoretical models developed provide a foundation for further refinement and scaling. Future work could involve adaptive control of the end effector based on real-time filament properties, integration with robotic vision for precise positioning, and testing on different safflower varieties. Additionally, the design principles could be adapted for other delicate crop harvesting applications, extending the utility of such end effectors in agriculture.
In conclusion, the double-acting opposite direction cutting end effector represents a significant advancement in safflower harvesting technology. Through meticulous design, theoretical analysis, and experimental optimization, we have developed an end effector that balances high filament removal rates with low breakage rates. The optimal parameters—cam speed 27.9 r/min, edge tilt angle 16.1°, and blade face inclination angle 19.7°—yield a removal rate over 91% and a breakage rate below 6%. This end effector not only addresses the key challenges in safflower harvesting but also provides a scalable and adaptable framework for the mechanized harvesting of other delicate crops. The continued evolution of such end effectors will play a crucial role in enhancing agricultural productivity and sustainability.