Jingyi Ma
Kun Wu
Ang Gao
Yonghui Du
Yonghui Du
Yuepeng Song
Yuepeng Song
1,3,* and
Longlong Ren
Longlong Ren
College of Mechanical and Electronic Engineering, Shandong Agricultural University, Tai’an 271018, China
Department of Traffic Engineering, Shandong Transport Vocational College, Weifang 261206, China Key Laboratory of Horticultural Machinery and Equipment of Shandong Province, Tai’an 271018, China Authors to whom correspondence should be addressed. Agriculture 2024, 14(8), 1302; https://doi.org/10.3390/agriculture14081302Submission received: 9 July 2024 / Revised: 30 July 2024 / Accepted: 6 August 2024 / Published: 7 August 2024
(This article belongs to the Section Agricultural Technology)Inspired by the maxillary mouthparts of longicorn beetles, four types of bionic cutters were designed in this research to address the prevalent issues of high cutting resistance and severe stubble damage encountered during alfalfa harvesting. Finite element simulation was utilized to assess the structural integrity and cutting performance of these bionic cutters. Additionally, bench tests were conducted on a homemade stem-cutting force measurement and control rig to evaluate their effectiveness. The results indicated: (1) the bionic cutters achieved a reduction in maximum equivalent force ranging from 20.9% to 49.2% and a decrease in maximum deformation from 31.4% to 64.1% compared to conventional cutters; (2) the maximum cutting resistance of alfalfa stems was reduced by 28.6%, 43.9%, 52.4%, and 38.6%, significantly enhancing the flatness of the cut surfaces; (3) orthogonal bench tests demonstrated that the type of cutter and the slip-cutting angle significantly influenced the maximum cutting resistance of the stems ( p < 0.01), with the optimal configuration being bionic cutter c, a slip-cutting angle of 10°, and a rotational speed of 2600 rpm. In conclusion, bionic cutters demonstrate substantial advantages in reducing maximum cutting resistance and improving the flatness of alfalfa stubble, suggesting their potential for widespread application and adoption.
Alfalfa, often hailed as the “king of forages”, is a high-quality perennial forage legume extensively cultivated around the globe. It serves as a vital source of digestible fibre, particularly beneficial for ruminants like dairy cows, not only providing protein for animal feed but also enhancing sustainable agriculture by contributing nitrogen to the soil when used as a rotational crop [1,2,3]. The design of the cutter is crucial in determining the quality of alfalfa stem harvesting and the extent of residual damage. Recent research has concentrated on refining harvester operations by optimizing cutter blade geometry to improve efficiency. Shiyu Song et al. explored the mechanical properties of sisal blades under rotary impact cutting conditions, successfully optimizing cutting parameters to diminish both cutting force and energy consumption [4]. Similarly, Hao Gan et al. assessed the impact of three distinct blade designs on energy consumption during the harvesting of manzanita grass, concluding that serrated blades outperform straight and slanted blades in reducing energy usage and enhancing the theoretical field capacity of operations [5].
Bionic cutters represent a significant innovation in agricultural machinery, enhancing equipment performance by mimicking the structures and functions of natural organisms, thereby becoming a crucial area of research in agricultural machinery [6,7,8,9,10]. Jinpeng Hu et al. explored the design of a rice straw cutting knife modelled after the East Asian flying locust, validating the bionic cutter’s effectiveness in reducing cutting resistance and energy consumption through discrete element method simulations [11]. Hongyan Qi et al. developed a biomimetic device for root stubble cutting inspired by the jaw structure of leaf-cutting ants, demonstrating its superiority over conventional blades in reducing cutting resistance and energy usage [12]. Zhengdao Liu et al. created a bionic blade for harvesting wild chrysanthemums based on the incisors of a cricket’s upper jaw, which significantly lowered shear force and operational energy consumption [13]. By emulating the structure and cutting patterns of locust mouthparts, researchers developed a bionic cutting device that not only enhanced stubble-cutting efficiency but also substantially reduced cutting torque and energy consumption [14,15]. In summary, bionic cutters are engineered by analyzing the geometrical characteristics of insect maxillary mouthparts and other animal foraging organs. This approach leverages nature’s efficiency to notably improve the performance of agricultural machinery components, making them more efficient and durable.
The upper jaw of the longicorn beetle is equipped with very sharp cutting teeth, capable of efficiently slicing through tough bamboo. This natural cutting prowess offers invaluable insights for bionic machine design. Tian Kunpeng et al. engineered both rotary-cutting and reciprocating bionic cutters, drawing inspiration from the unique cutting mechanisms of the longicorn’s maxillary mouthparts. These bionic cutters demonstrated lower cutting resistance and power consumption compared to traditional blades, thereby enhancing harvesting efficiency and quality and validating the efficacy of bionic cutters across different cutting modes [16,17]. In this paper, building on the alfalfa rotary harvesting cutter, we explored the drag reduction principle. We designed four types of bionic cutters, utilizing the white striped longicorn beetle as the bionic prototype. We conducted simulations and bench comparison tests to analyze the cutting process of alfalfa stems with both bionic and conventional cutters. The evaluation focused on comparing maximum cutting resistance, stubble levelling, and the maximum equivalent force exerted on the stems. Finally, through bench orthogonal tests, we identified key factors influencing the maximum instantaneous cutting resistance of the bionic cutter and determined the optimal parameter combinations.
The Whitestriped Longicorn Beetle, also referred to as the White-striped or Walnut Aspen Beetle, is depicted in Figure 1a and belongs to the Coleoptera Tenidae family [18]. Adults of this species typically measure between 34 and 65 mm in length and are predominantly dark brown or greyish brown and adorned with distinctive white spots on their wing sheaths that resemble clouds. As illustrated in Figure 1b, their mouthparts are of the masticatory type, exceptionally suited for piercing and cutting plant tissues. These mouthparts comprise two critical components: the impale segment and the cutting segment, which work collaboratively to process plant branches efficiently.
The curvature of the impaling segment, located on the inner side of the mouthpiece near the tooth tip, features a subtle change. This design preserves the sharpness of the tooth tip, significantly enhancing impaling efficiency. Moreover, the curve design reduces the tip diameter increment, thereby minimizing resistance during the piercing process, which constitutes about 1/5 to 1/6 of the entire medial curve. Directly adjacent to the piercing segment is the cutting segment, characterized by a concave curve and a convex “thin plate” structure akin to a cutting edge. This configuration not only prevents branches from slipping during impalement but also facilitates cutting during the process, thereby boosting mastication efficiency. Consequently, in this paper, the longicorn mouthparts were utilized as a bionic prototype for the alfalfa stem rotary cutter. This adaptation aims to enhance the cutter’s gripping performance and reduce both cutting resistance and sectional damage during harvesting, achieving low-damage harvesting.
This paper involved the selection of live Whitestriped Longicorn Beetles from local botanical gardens. After their natural demise, the maxillary mouthparts of the beetles were severed, rinsed with distilled water, and air-dried. The specimens were then examined under a stereo microscope (Axio Scope 5, Carl Zeiss AG, Oberkochen, Germany), as depicted in Figure 2. The inner teeth of the mouthparts (identified as the oa segment in Figure 2) were notably sharp. These teeth engaged branches and executed cutting motions during the beetle’s feeding process, mirroring the operational principle of a rotary cutter. Consequently, the internal curvature of the longicorn’s mouthparts was selected as the bionic model for designing a cutter suited for alfalfa stems.
The mouthpiece specimen’s image was processed using MATLAB 2021 software, as depicted in the flowchart in Figure 3. Initially, the image was converted to grayscale, followed by erosion and dilation to enhance the boundaries. It was then binarized to eliminate unnecessary data, and ultimately, the coordinate values of the edge contour points were extracted and saved. Finally, polynomials were fitted to the extracted 2D coordinate points using the curve-fitting toolbox in the MATLAB software. The fitted curves for the extracted points and tooth profiles on the medial side of the mouthparts of the longicorns are shown in Figure 4, and their curve-fitting 4-term polynomials are shown in Equation (1), where the fitting variance, R 2 , is 0.995.
f x = 0.006315 − 0.4328 x + 0.009996 x 2 + 0.01843 x 3 − 0.001123 x 4The complexity of the structure in forage harvesting cutters correlates with increased production costs, which is why the blades in most combine harvester cutting devices are predominantly linear flat cutters [19]. Although economical and straightforward, these linear flat cutters often lack sufficient clamping force. When these high-speed rotating flat cutters strike the surface of the forage stems, they frequently cause the stems to slip [20], leading to poor cut quality and more significant damage to the sections. Consequently, drawing on the commonly used structure of forage cutter blades, as depicted in Figure 5, where the overall dimensions are in millimetres, the bionic cutter design utilizes the inner curve of the longicorn mouthparts as the blade edge. The performance of this bionic cutter is further analyzed using the finite element method.
Considering the influence of the cutter edge size on the cutting performance and quality, four bionic cutter models were designed by scaling the inner curve of the original longicorn mouthpiece, namely, the use of multiple original-size curves, a 0.75-fold curve, and a 0.5-fold curve, applying multiple curves head-to-tail to the cutter edge, as shown in Figure 6a–c below, respectively, and enlarging a single curve to be of the same length as the original cutter. As shown in Figure 6d below, the names of these four bionic cutter models are simplified as bionic cutters a, b, c, and d.
Theoretical analysis of the cutting resistance for both conventional and bionic cutters, conducted using a graphical method, is illustrated in Figure 7. In this figure, F represents the combined force exerted by the cutter on the alfalfa stem. At the same time, Fn and Ft denote the component forces perpendicular and parallel to the cutting edge, respectively. The interrelationship between these three forces is detailed in Equation (2).
F 2 = F n 2 + F t 2As illustrated in Figure 7a, the conventional cutter exerts only a force F perpendicular to the cutting edge on the alfalfa stems. Consequently, during the cutting process, the stems move solely in the direction of force F without any slippage along the cutting blade. This results in no gripping effect on the stems by the conventional cutter and higher cutting resistance. In contrast, as shown in Figure 7b, the force Ft along the cutting surface direction pushes the alfalfa stem towards the left cutting edge, enhancing the sliding cutting effect and reducing the cutting resistance. Moreover, when the tooth blade is smaller, more force contact points occur during the cutting of alfalfa stems. Theoretically, the smaller the scale of the bionic curves, the greater the number of cutting force points [11]. However, the production cost of the cutter also increases. Therefore, in this paper, only curves scaled to 1, 0.75, and 0.5 times were utilized for the tooth blade of the bionic cutter.
We established solid models of five different cutters using SolidWorks 2022 software. Subsequently, we imported them into the finite element analysis ABAQUS 2020 software for tasks such as setting material properties, defining boundary conditions, and mesh delineation. The cutter’s material properties are critical to the overall efficiency of the machinery and equipment. For this paper, the cutter matrix material was chosen as 65 Mn metal, with its material properties detailed in Table 1.
In alignment with the actual operational conditions, the circular hole on the cutter is fixed above the cutter disk. It rotates synchronously with the cutter disk, resulting in the internal settings of the circular hole being completely immobilized (U1 = U2 = U3 = UR1 = UR2 = UR3 = 0). During operation, the cutter primarily encounters the reaction force from the alfalfa stems; hence, in this paper, we disregarded the frictional resistance between the cutter and the air, focusing solely on the impact of the alfalfa stems on the cutter. The collision between the cutter and the stem is a complex interaction process, and it is currently not possible to accurately use finite element simulation to model the forces on the cutter during dynamic cutting of alfalfa stems. However, the force on the cutter during static cutting of stalks is much greater than that during dynamic cutting. Therefore, this study uses static cutting resistance instead of dynamic cutting resistance to investigate the limit case of the structural strength of the cutter. A shear test conducted on alfalfa stems approximately 3 mm in diameter using a texture meter (EX-LX SET ASSY CH, Shimadzu Corporation, Kyoto, Japan) indicated a shear force of about 100 N. Consequently, a uniform load of 100 N was applied to the cutting surface of the cutter.
Given that crop stems, including those of alfalfa, are predominantly viscoelastic and anisotropic materials [21,22], the mechanical properties of alfalfa stems have been simplified to those of transverse isotropic materials to facilitate more straightforward calculation. Moreover, the vascular bundles in the alfalfa stem resemble the longitudinal fibres found in composite materials. Consequently, the damage criterion for alfalfa stems can be based on the Hashin damage criterion, which is commonly applied to composite fibre materials [23,24].
The 2D Hashin criterion implemented in ABAQUS is capable of describing composite damage under 2D in-plane stress states, but it needs to predict progressive damage in 3D stress states. In practice, the behaviour of composite damage under 3D stress conditions more accurately reflects actual damage [25]. The expressions for the 3D Hashin damage mechanism are detailed in Equations (3)–(7), where the satisfaction of any failure mode results in the immediate deletion of the unit. Consequently, this paper employs the three-dimensional Hashin damage subroutine to characterize the destructive damage of alfalfa stems.
Based on the previous work of [26], axial compression, radial compression, bending, and shear tests were performed on alfalfa using a texture meter (EX-LX SET ASSY CH, Shimadzu Corporation, Kyoto, Japan) to calculate the parameters of alfalfa stems as an anisotropic material property. The specific material property parameters for the alfalfa stem are detailed in Table 2, and the workflow of the 3D Hashin simulation model is depicted in Figure 8.
