Design and Evaluation of a Mechanical Floor-Standing Jujube Picker

Abstract

The poor adaptability of existing harvesting machinery and low work efficiency is observed due to the low-level mechanized harvesting of Xinjiang jujube orchards. A mechanical floor-standing jujube picker was designed on the basis of the characteristics of the high fruit drop of the Xinjiang jujube harvest period. The machine can pick up the jujubes, which have been manually collected into strips on the ground. The structure and working parameters of the pickup device are determined in accordance with the pickup principle. The conditions for satisfying the picking operation were analyzed, and the range of motion speed ratio parameters to meet the pickup operation was obtained. Furthermore, the contact process of jujubes on the conveying and separating device was analyzed and studied; the factors affecting the collision damage and conveying the balance of jujubes were analyzed; and the basic structural parameters of the conveying and separating devices were determined. A mechanical floor-standing jujube picker was also fabricated for trial. A three-factor and three-level Box– Behnken test was performed by taking the forward speed of the machine, the speed of the strip brush roller shaft, and the hole pitch height of the profiling rod as factors, and the pickup, breakage, and impurity rates of jujubes as indicators. Results show that when the forward speed of the machine was 0.3 m·s⁻¹, the speed of the strip brush roller shaft was 53 rpm and the hole pitch height of the profiling rod was 60 mm. The jujube pickup rate of the machine was 92.11%, the breakage rate was 2.07%, and the impurity rate was 4.15%. The relative error with the prediction model was less than 8%. Thus, the model is reliable and meets the operational requirements. This study can provide a reference for the mechanical floor-standing jujube picker.

1. Introduction

Jujubes have a long history of planting [1] and are one of the unique fruit trees in China [2]. As one of the six characteristic forest and fruit industry belts in the world, the characteristic forest and fruit industry of Xinjiang has already become a key supporting industry for adjusting the industrial structure and increasing income in Xinjiang [3]. During the harvest period, the connection between the jujube stalk and the jujube weakens, and a large amount of jujube fruit falls off due to external environment disturbances, resulting in fallen jujubes. The high fruit drop rate means that the main method of harvesting Xinjiang jujubes is manually picking up the fallen jujubes. However, manual harvesting encounters problems such as high operation costs, low efficiency, and labor shortages [4,5,6]. Although mechanized picking can replace manual operations and reduce operating costs, the complex structure, the difficulty in adjusting operation parameters, the need for multiple manual laborers to assist in the operation, and low versatility are key problems restricting mechanized harvesting operations. Therefore, it is of great significance to develop a mechanical floor-standing jujube picker with a simple structure, easy operation, strong versatility, and stable performance.

Jujubes are mostly planted as ornamental fruit trees in foreign countries, and studies on jujube harvesting machinery are relatively few [7]. Sumner and Hedden [8] designed a combination rake-pickup machine, which can pick up citrus and other hard forest fruits. The citrus harvester developed by OXBO [9] International Specialty Crop Agricultural Machinery Company mainly picks up citrus and other fruits with hard peels and large volume. The current domestic research on Xinjiang jujube harvest mainly focuses on the picking direction of landing jujubes considering the jujube harvest problem in Xinjiang. Jujube pickers can be divided into the following two categories considering different harvesting principles: pneumatic and mechanical jujube pickers. Zhang et al. [10] developed a pneumatic jujube harvester, in which the operator picks up the jujubes on the ground while holding a suction pipe. The suction chamber then cleans and separates the jujubes from impurities to complete the picking task. Pan et al. [11] developed an air-blowing type ground jujube pickup device, which uses positive pressure to blow the jujube to the conveying device, transport it backward for storage, and complete the operation. The sweeping jujube harvester developed by Lu Bing et al. [12] collects the ground jujube into strips through a V-shaped arrangement of sweeping sticks, and then the stick brush sweeps the jujube onto the conveyor to complete the harvesting. Wang et al. [13] developed a comb-tooth-type floor-standing jujube picking device to pick up the fallen jujubes through the comb-tooth scraping operation principle. The picked-up jujubes are then transported to the jujube box by the conveying device to complete the picking operation. From the above viewpoint, the characteristics of fruits picked up by foreign forest fruit harvesting machines are quite different from those of jujubes, which are unsuitable for picking up jujubes. Although the picking effect of the pneumatic jujube harvester is excellent, its versatility is poor. Moreover, it has high environmental requirements, the fan can quickly become worn by sand, and its performance is not sufficiently stable. Mechanical jujube pickers have a relatively high working efficiency and low operating environment requirements. However, there are some problems such as difficulties in adjusting the operation parameters, a complex structure, and the need for many manual auxiliary operations. Thus, it is very important to optimize such farm machinery according to the local conditions and human resources [14,15].

Therefore, this study takes into consideration the analysis of the above jujube picking machinery combined with the characteristics of the high fruit drop of Xinjiang jujubes during the harvest period. In this study, a new mechanical floor-standing jujube picker for picking up jujubes that have been manually collected into strips on the ground was designed based on the principle of comb teeth entering the soil and picking up the jujubes with the strip brush. The structural parameters of the machine were determined according to the operation requirements, and the basic operation parameters were obtained by theoretical analysis. The Box–Behnken method was used to analyze the performance of the mechanical floor-standing jujube picker. This paper can provide a new harvesting structure and theoretical reference for the mechanical floor-standing jujube picker.

2. Materials and Methods

2.1. Machine Structure

The mechanical floor-standing jujube picker is mainly composed of a pickup device, a conveying and separating device, a fruit collecting device, and a hydraulic power system. It is mainly used to pick up the ground jujube manually collected in the middle of the jujube trees. The entire machine can complete the pickup, conveying impurity removal, fruit collection, and other operations simultaneously. The structure of the entire machine is shown in Figure 1, and the structural parameters are presented in

Table 1. Structural parameter table of mechanical landing red date pickup machine.

Parameters	Values
Structure form	Self-propelled
Fuselage steering mode	Hydraulic type
Overall dimension (length × width × height)/mm	4270 × 2200 × 2017
Engine rated power/kW	38
Engine rated speed/(r·min−1)	2400
The whole machine quality/kg	1600
Effective working width/mm	1150

.

2.2. Operating Principle of the Entire Machine

The machine is powered by the engine and drives the hydraulic pump to supply oil to the entire hydraulic system. A hydraulic motor drove the crawler chassis. During the operation, the jujubes, jujube branches, jujube leaves, sand, and other materials collected manually into strips on the ground (hereinafter collectively referred to as strips) are firstly piled up and climbed along the comb teeth, and the strip brush combs and picks up the piled materials. The picked jujubes are cleaned by an axial flow fan installed on the side of the conveying and separating device and then transported to the fruit collecting device for temporary storage by the conveying and separating device. Finally, the fruit collecting device pours out the temporarily stored jujubes by overturning the oil cylinder for centralized processing and completes the picking and harvesting of the jujubes on the ground.

2.3. Design of the Pickup Device

2.3.1. Structural Design of Pickup Device

The pickup device is the core part of the entire machine, and its function is to pick up jujubes and partially remove impurities. The pickup device is mainly composed of the comb teeth, pickup side plate, strip brush, profiling wheel, and hydraulic lifting pulley rod (Figure 2). The pickup device is installed between the crawler chassis and is at the bottom of the driver’s seat. The operating depth and angle of the comb teeth are adjusted by modifying the height between the profiling rod hole and the fixed plate (hereinafter referred to as the hole pitch height of the profiling rod). During the operation, the hydraulic motor provides power to drive the strip brush to transfer back.

The lifting rod is controlled by the hydraulic cylinder and is responsible for lifting and lowering the pickup device. In order to prevent the ground from undulating and bumping the comb teeth, when the whole machine is not picking up and only walking, the lifting rod can raise the end of the comb teeth to a distance of 0~200 mm from the ground. When the whole machine is in a working state, the rolling pulley on the lifting rod rolls along the picking side plate so that the pickup device descends along the arc track. After the comb teeth are buried in the soil, the rolling pulley drives the lifting rod to continue to descend along the arc until it does not interfere with the pickup operation.

2.3.2. Determination of Structural Parameters of the Pickup Device

The field measurement revealed that the width of jujube belts on the ground after manual collection and accumulation in the harvest period of jujube gardens in southern Xinjiang is less than 1000 mm. Considering the deviation of the actual machine travel path, the actual machine travel deviation is taken as 20–50 mm, and the effective working width of the pickup is 1100 mm.

As the core working parts of the pickup device, the comb teeth and their angle entering the soil will directly determine whether jujubes can accumulate and climb along the comb teeth. Figure 3 illustrates the force diagram of jujube on the comb teeth in an ideal state. The force analysis shown in Figure 3 indicates that the balance condition of jujube on the comb teeth is

$$F\cos \theta =F_f+G\sin \theta$$

(1)

$$F_N=G\cos \theta +F\sin \theta$$

(2)

Equations (1) and (2) are combined to obtain the following:

$$F\ge G\tan \left(\theta +\psi \right)$$

(3)

According to Equation (3), the climbing force of the jujube along the comb teeth is affected by its gravity, the angle of the comb teeth entering the soil, and the friction between the jujube and comb teeth. The soil breaking capability is gradually enhanced with the increase in the angle of the comb teeth entering the soil; however, this increase is not conducive to the accumulation and climbing of jujubes on the comb teeth. The comb angle is 30°–33° according to the pre-test of the researchers and the reference [16]. They found that within the range of the comb teeth angle entering the soil, the jujubes can still be shoveled, the change in the angle has minimal influence on the picking effect, and the picking effect is mainly affected by the depth of the comb teeth entering the soil. Simultaneously, the depth of the comb teeth entering the soil determines the soil hilling quantity. A certain amount of soil hilling quantity is conducive to the accumulation and climbing of jujubes along the comb teeth, but excessive soil hilling quantity easily buries jujube in the soil and reduces the work efficiency. The depth of the comb teeth entering the soil is also different under the condition of various soil types and land flatness. The jujube garden in Southern Xinjiang is dominated by sandy land. Sandy soil has good fluidity but is easily disturbed by machines during operation; through the measurement of land flatness, the researchers determined that the depth of the comb teeth entering the soil h₁ is 0–40 mm based on the actual terrain of the jujube garden [17]. The depth of the comb teeth entering the soil is adjusted by modifying the hole pitch height of the profiling rod (Figure 4). The initial state is assumed to be comb teeth depth entering the soil at 40 mm and the angle of the comb teeth entering the soil at 33° to determine the parameter range of the hole pitch height of the profiling rod. The comb teeth rotate from the comb teeth depth, entering the soil from point A₁ to point A₂ along the center J of the pickup drive shaft. When the hole pitch height of the profiling rod is not adjusted, the center O of the profiling wheel moves to point Q; at this time, comb teeth depth entering the soil is 0. It is known from the geometric relationship in Figure 4.

$$\angle JOH=\arcsin \frac{L_{JD}-R}{L_{JO}}$$

(4)

$$L_{J{H}_1}=L_{JO}\sin \left(\angle JOH-\beta \right)$$

(5)

$$L_{H{H}_1}=L_{JH}-L_{J{H}_1}$$

(6)

where R is the diameter of profiling wheel, mm; L_JD and L_JO are the vertical distance from the center J of the pickup driving shaft to the ground and the distance from the center of the pickup driving shaft to the end of the comb teeth, mm; β is the Angle at which the comb teeth rotate from A₁ to A₂ along the center of the rotating axis J, (°).

Equations (4)–(6) indicate that when the depth of the comb teeth entering the soil is 0 and the profiling wheel contacts the ground normally, the hole pitch height of the profiling rod L spacing should be:

$$L=L_{QG}+h_2=\frac{L_{H{H}_1}}{\cos \beta }+h_2$$

(7)

where h₂ is the hole pitch height of the profiling rod in the initial state, mm.

Where R takes 350 mm, and L_JD and L_JO are designed by the entire machine size of 815 and 1235 mm, respectively. β is solved by the geometric relationship within Figure 4 to 2.53°, and h₂ takes 21 mm. Therefore, L obtained from Equation (7) is 76.5 m. At this time, the comb teeth angle entering the soil is slightly larger than 30°. The depth of comb teeth into the soil is generally greater than 0 in actual work; thus, the actual value L is 75 mm. The value range of the hole pitch height of the profiling rod is 21–75 mm. The clearance of comb teeth is determined by the shaft diameter of the jujube. Based on the data in [17], the value range of the shaft diameter of the jujube is 19.23–41.53 mm; therefore, the clearance of comb teeth is set as 18 mm.

2.3.3. Analysis of Conditions Meeting the Requirements of the Jujube Picking Operation

One of the basic conditions for meeting the operation of the pickup device lies in the reasonable range of values of the speed ratio of the strip brush roller shaft line to the forward speed of the implement λ. The interaction between sand and jujubes and the scattering of jujubes is ignored to simplify the calculation and facilitate the analysis. The current study proposes two climbing state hypotheses based on an actual operation. Assume climb state I (Figure 5a). The machine advances at a uniform speed of V₀, and the jujubes climb along the comb teeth. The climbing state of the jujube is the same as the stacking state M on the ground, and the accumulation thickness is h. The accumulation state and thickness of the jujubes along the comb teeth climb are also assumed to remain unchanged. Within t time, the end of the strip brush touches the jujubes at point A₁, picks them up, and turns them to point B₁. Figure 5b shows climb state II. The machine advances at a uniform speed of V₀. The scattered jujubes are assumed to be under a single grain state during the accumulation and climbing along the comb teeth in the form of a single layer M₁. Within time t, the strip brush contacts the jujubes at point C₁ and then picks them up.

For climb state I, the strip brush successfully picks up the jujubes when their obtained range by the machine within the time t can be completely combed by the strip brush. A jujube is granular, thus endowing it with fluidity. Therefore, within time t, the jujubes have not scattered from the comb teeth and still maintained the accumulation state on the ground. The forward distance of the machine is approximately equal to the climbed distance of the jujubes along the comb teeth. Figure 5a and theoretical analysis revealed that

$$\{\begin{array}{l}S=V_{0}t\\ \phi =\arccos \left(\frac{R-h}{R}\right)\\ S^2+{\left(R-h\right)}^2=R^2\end{array}$$

(8)

where S is the distance traveled by machines within t time, m; V₀ is the forward speed of the machines, m·s⁻¹; φ is the angle of brush rotation in t time, (°); h is the height of jujubes in the state of ground accumulation after manual strip collection, mm; R is the distance from the center point of the strip brush roller shaft to the end of the strip brush, mm.

By solving Equation (8) can be obtained

$$t=\frac{\pi \arccos \left(\frac{R-h}{R}\right)}{180\omega }=\frac{\sqrt{2Rh-h^2}}{V_0}$$

(9)

$${\omega }_{\min }=V_0\frac{\pi \arccos \left(\frac{R-h}{R}\right)}{180\sqrt{2Rh-h^2}}$$

(10)

Climb state II is the same as climb state I. Within time t, the volume of jujubes swept by the machine should be equal to that of jujubes accumulated on the comb teeth surface. Therefore

$$\{\begin{array}{l}{V}_a=V_{0}thb=Shb\\ V_b=h_{2}Lb\\ V_a=V_b\end{array}$$

(11)

where L is the length of jujubes laid on the comb teeth surfaces in a single grain state, m; b is the working width, m; V_a is the volume of jujubes swept during the advance of machines in t time, m³; V_b is the volume of jujubes accumulated on the comb teeth surface in t time, m³. h₂ is the average length of the longitudinal axis of single jujube, mm.

It can be seen from the geometric relationship in Figure 5b that

$$\{\begin{array}{l}{\phi }_2=\arctan \left(\frac{L}{R-h_2}\right)\\ L^2+{\left(R-h_2\right)}^2=R^2\\ t=\frac{\pi \arctan \left[\frac{\sqrt{R{h}_2-h_2{}^2}}{\left(R-h_2\right)}\right]}{180\omega }=\frac{h\sqrt{R{h}_2-h_2{}^2}}{V_0{h}_2}\end{array}$$

(12)

where φ₂ is the rotation angle of the strip brush in t time, (°).

Then the simultaneous Formulas (11) and (12) get the speed of the strip brush as

$${\omega }_{\max }=V_0\frac{\pi h\arctan \left[\frac{\sqrt{R{h}_2-h_2{}^2}}{\left(R-h_2\right)}\right]}{180h_2\sqrt{R{h}_2-h_2{}^2}}$$

(13)

Pickup speed ratio λ by

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

(14)

where V₀ is taken as 0.1–0.5 m·s⁻¹ according to the operation requirements, h is the stacking height ranging from 30 mm to 150 mm according to field measurements, and h₂ is 34 mm according to reference [16]. R is determined by references [13] and [18], and [19], and the length is 225 mm. The pickup speed ratio λ is 1.2–4.6 based on Equations (10), (13) and (14). The pickup speed ratio λ should be slightly larger than the theoretical value due to the interaction between sand and jujubes in practical operation and the scattering of jujubes. Therefore, the rotation speed of the strip brush roller shaft is selected as 45–65 rpm according to the above analysis.

2.4. Conveying and Separating Device

2.4.1. Structural Design of the Conveying and Separating Device

The conveying and separating device is mainly composed of a conveying drive shaft, supporting wheel, guide pulley, conveying side plate, pickup driving bearing seat, conveying combination rod, plum blossom wheel, and axial flow fan (Figure 6). The conveying and separating device is located in the middle of the hydraulic track and is hinged with the pickup device. The conveying and separating device receives the jujube sand mixture picked up by the strip brush. The conveying combination rod then conveys the jujubes to the fruit collecting device at the rear end. Some jujube branches, jujube leaves, and sand fall from the clearance of the conveying combination rod under the micro-vibration action of the machine. The remaining impurities will be separated from the jujubes under the action of the fan.

The conveying and separating device is based on the undulating fence conveying mechanism, and the rod is designed as the undulating conveying combination rod. The clearance of the conveying combination rod should be less than the minimum shaft diameter of the jujubes. The minimum shaft diameter of the jujube is 19.23 mm based on the previous article, and the clearance of the conveying combination rod is 18 mm. The impurities in the transportation process are mainly jujube branches and jujube leaves because the sand falls from the clearance between the comb teeth under the action of the strip brush. Therefore, air blowing was used to separate jujubes from jujube branches and leaves. The jujubes cannot be blown off; thus, the air velocity of the fan should be larger than the impurities and less than the critical velocity of the jujubes to meet the required wind force for cleaning [20]. The critical velocity of impurities is 3.4–5.6 m·s⁻¹, and that of jujubes is 27.13–34.28 m·s⁻¹, according to the literature [20,21,22]. Therefore, the YWF2E-300 axial flow fan with an air volume of 3300 m³·h⁻¹ was selected on the basis of the critical velocity of the jujubes and impurities [23]. The blowing angle of the fan is adjusted by the fan base on one side of the lifting rod of the material box. The blowing angle is 70°–90°. The specific parameters of the conveying and separating device are shown in

Table 2. The conveying and separating device parameters.

Parameters	Values
Length of conveying side plate/mm	1900
Conveying width/mm	1280
Driving shaft speed of conveying/r·min−1	11~185
clearance of conveying combination rod/mm	18
Fan air volume/m3·h−1	3300

.

2.4.2. Analysis of the Contact Process between the Jujubes and Conveying Combination Rod

The jujubes are conveyed by the conveying combination rod after being brushed from the pickup comb teeth to the conveying and separating device. During the working transportation of the jujubes and the conveying combination rod, two contact processes of instantaneous collision and extrusion will emerge at the beginning, and the rotating conveying with the conveying combination rod will be in a balanced state. Amongst them, instantaneous collision and extrusion are one of the main reasons for the damage of jujubes during the entire harvest operation. Therefore, the two processes of instantaneous collision extrusion and rotary conveying in an equilibrium state are analyzed and studied, respectively. Taking the center of mass of the jujube as the origin, the coordinate system is established, and the jujube flies out at the end of the comb teeth under the action of a strip brush. Ignoring air resistance, jujubes move at a uniform speed in the horizontal direction (Figure 7). In the vertical direction, the jujubes move to the highest point, h₁, at a uniform deceleration and then fall on the conveying combination rod at a uniform acceleration after reaching the highest point. Jujubes also collided with the conveying combination rod.

During the instantaneous collision between the jujubes and the conveying combination rod, the line between the throwing starting point and falling point is approximately horizontal. The highest point h₁ and the horizontal displacement L_X that the jujubes can reach are

$$h_1=\left(V_1\sin \theta \right)t_1-\frac{1}{2}g{t}_1{}^2$$

(15)

$$L_X=V_{2X}t=L_H\cos \beta$$

(16)

where θ is the angle of the comb teeth entering the soil, (°); t₁ is the time taken for jujubes to reach the highest point. β is the angle of installation angle of the conveying and separating device, (°). L_H is the distance between comb teeth and conveying and separating device, m.

Simultaneous Equations (15) and (16) can obtain the instantaneous closing speed of the collision between the jujubes and the conveying combination rod as follows:

$$V_2=\sqrt{{\left(V_1\cos \theta \right)}^2+{\left[g\left(\frac{L_H\cos \beta }{V_1\cos \theta }-\frac{V_1\sin \theta }{g}\right)\right]}^2}$$

(17)

Jujube is an elastic–plastic material. The damage caused by the collision is mainly due to the extrusion deformation caused by the collision in the normal direction [24,25,26,27]. The friction between the rods and jujubes is ignored. The secondary collision caused by a small part of the jujubes after the collision of the rods is disregarded, and only the collision deformation caused by the normal compression is studied. The conveying combination rod is regarded as a plane, and the contact model is simplified as the contact and collision between the jujube and plane. Using the Hertz theory [28], the normal force acting on the contact area according to contact mechanics is as follows:

$$F=\frac{4}{3}{E}^{\ast }\sqrt{R^{\ast }}{\delta }^{\frac{3}{2}}$$

(18)

The equivalent elastic modulus and equivalent radius of the collision satisfy, respectively,

$$\{\begin{array}{l}\frac{1}{E^{\ast }}=\frac{1-{\mu }_1{}^2}{E_1}+\frac{1-{\mu }_2{}^2}{E_2}\\ \frac{1}{R^{\ast }}=\frac{1}{R_1}+\frac{1}{R_2}\end{array}$$

(19)

where E* is the equivalent elastic modulus; E₁ and E₂ are the elastic modulus of the jujube and conveying combination rod; μ₁ and μ₂ is the Poisson’s ratio of the jujube and conveying combination rod; R* is the equivalent radius, mm; R₁ and R₂ are the radii of the jujube and conveying combination rod, because the model is equivalent to the contact between the jujube and plane, R₂ is about infinity, mm.

The relationship between compression deformation and the yield stress of the jujubes [29] is

$$\{\begin{array}{l}{\delta }_Y={\left(C{\sigma }_Y\right)}^2{\left(\frac{\pi \sqrt{R^{\ast }}}{2E^{\ast }}\right)}^2\\ C=\min \left\{1.295e^{0.763{\mu }_1},1.295e^{0.763{\mu }_2}\right\}\end{array}$$

(20)

Substituting Formula (20) into (18), the yield pressure of jujubes is

$$F_Y=\frac{R^{\ast 2}{\left(\pi C{\sigma }_Y\right)}^3}{6E^{\ast }}$$

(21)

Combined with the mechanical properties of the jujubes, and referring to the relevant literature [30], the jujubes’ ability to resist deformation is the worst at the end of ripening. The rupture force of jujubes is approximately equal to the yield pressure of jujubes, 147.5 N, and the elastic modulus is 0.16 Mpa. The approximate yield stress and compressive deformation of jujubes can be obtained from the above Equations (20) and (21). According to the analysis of the collision process of jujubes and the law of energy conservation, the energy of collision loss is

$$W_S=\frac{1}{2}m{V}_{2Y}{}^2-W_Y$$

(22)

The work performed by the yield pressure W_Y is

$$W_Y=\frac{R^{\ast 3}{\left(\pi C{\sigma }_Y\right)}^5}{24E^{\ast 4}}$$

(23)

W_S should be close to 0 to decrease the damage to jujubes due to collision. After the collision compression process between the jujubes and the conveying combination rod is completed, the jujubes are rotated and conveyed with the conveying combination rod in a balanced state. The force of jujubes on the conveying combination rod is shown in Figure 8. Therefore, the balance condition of jujubes on the conveying combination rod is as follows.

Then the balance condition of jujubes in the horizontal and vertical direction of the conveying combination rod is

$$\{\begin{array}{l}{F}_{N1}\cos \gamma +F_{N2}\cos \tau =G\\ F_{N1}\sin \gamma =F_{N2}\sin \tau \\ F_{N1}\left(R_1+R_2\right)\sin \left(\gamma +\tau \right)-G\left(R_1+R_2\right)\sin \tau =0\end{array}$$

(24)

It can be seen from Figure 7 and geometric relationship

$$\{\begin{array}{l}{\psi }_1=\arccos \frac{L_G}{2\left(R_1+R_2\right)}\\ \gamma =\frac{\pi }{2}-\left({\psi }_1-{\varphi }_1\right)=\frac{\pi }{2}-\left(\arccos \frac{L_G}{2\left(R_1+R_2\right)}-{\beta }_2\right)\end{array}$$

(25)

When the mechanical properties, quality, speed of the strip brush roller shaft, rod diameter of the conveying combination rod, the material of the conveying combination rod, and other factors of jujubes are determined through Equations (17)–(25). The jujube damage is mainly related to the instantaneous collision speed between the jujubes and rod and the installation angle of the conveying and separating device. The main factors affecting the stability of jujubes in the conveying state are the angle between the conveying combination rod and the horizontal plane and the installation angle of the conveying and separating device. Overall, a large installation angle of the conveying and separating device leads to a short falling distance of jujubes and their low collision damage. However, the jujubes easily slip off the conveying combination rod in an unstable conveying state. On the contrary, the installation angle of the conveying separation device is too small, which will increase the overall structure size [31] and aggravate the damage to jujubes caused by collision. Based on the “Agricultural Machinery Design Manual” [32], the installation angle of the conveying and separating device is determined β to be 24°, and the included angle between the conveying combination rod and the horizontal plane β₂ is 8°.

2.5. Test Materials

A jujube harvesting test was conducted in the trunk jujube demonstration base of Kunlun Mountain Jujube Industry Co., Ltd., Kunyu City, China, 224 regiment of the 14th division of Xinjiang production and Construction Corps, China on November 2021. The demonstration base adopts the planting mode of 4 m × 1.5 m (row spacing × plant spacing). The test object was the ground jujubes after manual collection, and the width after the manual collection was between 50 cm–80 cm. The fruits were 4-year-old Jun jujubes, with a moisture content of 10.12–15.73%.

2.6. Test Methods

According to the test method of DG/T 188-2019 “Fruit Picker” [33], every 20 m of length between the rows of the jujube trees was taken as the test area. Before the test, the jujubes and impurities in the compartment should be removed, and the working prototype should be debugged according to the parameters required for the test before commencing the test.

The pickup rate, breakage rate, and impurity rate of the jujubes were used as evaluation indexes. Among these, the pickup rate is the mass ratio of jujubes effectively picked up in the test area to total jujubes. The breakage rate is the proportion of damaged jujubes among the pickup jujubes. The impurity rate is the ratio of the quality of the remaining materials to the total picked materials after removing the quality of the picked jujubes. The calculation methods for the pickup rate, breakage rate, and impurity rate are as follows:

$$Y_1=\frac{M_1}{M_2}\times 100\%$$

(26)

$$Y_2=\frac{M_3}{M_1}\times 100\%$$

(27)

$$Y_3=\frac{M_4-M_1}{M_4}\times 100\%$$

(28)

where Y₁ is the pickup rate, %; M₁ is the mass of jujubes effectively picked up in the test area, kg; M₂ is the total mass of jujubes in the test area, kg; Y₂ is the breakage rate, %; M₃ is the total mass of damaged jujubes among the effectively picked up jujubes in the test area, kg; Y₃ is the impurity rate, %; M₄ is the quality of total materials effectively picked up in the test area (including jujubes, jujube branches, jujube leaves, etc.), kg.

According to the structural design of the whole machine, the previous theoretical analysis, and the previous preliminary experiments, it can be known. The forward speed of the machine, the speed of the strip brush roller shaft, and the hole pitch height of the profiling rod have a great impact on the pickup operation effect. In order to ensure the pickup effect and improve operational efficiency, the following test parameters were used: the forward speed of the machine was 0.2–0.4 m/s, the speed of the strip brush roller shaft was 45–65 rpm, and the hole pitch height of the profiling rod was 21–75 mm. According to the Box–Behnken response surface design theory [34,35], the pickup rate Y₁, the breakage rate Y₂, and the impurity rate Y₃ were used as the response values. We conducted three-factor and three-level response surface tests. The test factors and levels are shown in

Table 3. Test factors and levels.

Levels	Forward Speed of the Machine X1/(m·s−1)	Speed of the Strip Brush Roller Shaft X2/(rpm)	The Hole Pitch Height of the Profiling Rod X3/mm
−1	0.2	45	21
0	0.3	55	48
1	0.4	65	75

.

2.7. Test Results

Using Design-Expert 11.0 software, we designed a three-factor and three-level Box–Behnken test. The experiment included 12 groups of analysis factors and five groups of zero error, for a total of 17 groups of experiments. The test scheme and results are shown in

Table 4. Test design and results of pickup jujubes on the ground.

No.	X1	X2	X3	Pick Up Rate Y1/%	Breakage Rate Y2/%	Impurity Rate Y3/%
1	−1	−1	0	92.1	1.61	4.45
2	1	0	1	92.27	1.83	3.35
3	0	1	1	96.52	3.65	3.42
4	0	0	0	95.27	2.16	4.93
5	−1	0	−1	93.68	2.4	7.89
6	0	1	−1	96.9	3.61	9.75
7	−1	1	0	94.43	3.8	6.21
8	1	0	−1	91.26	1.98	8.34
9	0	−1	−1	93.51	1.44	7.21
10	0	0	0	95.44	2.01	5.02
11	1	−1	0	89.55	1.56	4.76
12	0	−1	1	91.33	1.96	3.13
13	−1	0	1	90.3	2.63	3.26
14	0	0	0	95.39	2.09	4.86
15	0	0	0	96.15	1.97	5.17
16	1	1	0	95.86	3.17	6.37
17	0	0	0	95.92	1.92	4.91

.

3. Result and Discussion

We used Design-Expert 11.0 software (Stat-Ease Inc., Minneapolis, MN, USA) to analyze the test results and the multiple regression fit, as shown in

Table 5. Analysis of variance of regression equation.

	Simulated Item	Sum of Squares	Degree of Freedom	Mean Square	F-Value	p-Value	Significance
Y1	Model	86.50	9	9.61	105.32	<0.0001	***
	X1	0.3081	1	0.3081	3.38	0.1087
	X2	37.07	1	37.07	406.17	<0.0001	***
	X3	3.04	1	3.04	33.29	0.0007	***
	X1X2	3.96	1	3.96	43.40	0.0003	***
	X1X3	4.82	1	4.82	52.80	0.0002	***
	X2X3	0.8100	1	0.8100	8.88	0.0205	**
	X12	29.98	1	29.98	328.49	<0.0001	***
	X22	0.0016	1	0.0016	0.0171	0.8996
	X32	4.99	1	4.99	54.64	0.0002	***
	Residual	0.6388	7	0.0913
	Lack of fit	0.0611	3	0.0204	0.1410	0.9303
	Pure error	0.5777	4	0.1444
	Cor total	87.14	16
Y2	Model	9.14	9	1.02	69.47	<0.0001	***
	X1	0.4512	1	0.4512	30.88	0.0009	***
	X2	7.33	1	7.33	501.87	<0.0001	***
	X3	0.0512	1	0.0512	3.50	0.1034
	X1X2	0.0841	1	0.0841	5.75	0.0475	**
	X1X3	0.0361	1	0.0361	2.47	0.1600
	X2X3	0.0576	1	0.0576	3.94	0.0875	*
	X12	0.0026	1	0.0026	0.1801	0.6841
	X22	0.9701	1	0.9701	66.38	<0.0001	***
	X32	0.1012	1	0.1012	6.92	0.0339	**
	Residual	0.1023	7	0.0146
	Lack of fit	0.0657	3	0.0219	2.39	0.2091
	Pure error	0.0366	4	0.0091
	Cor total	9.24	16
Y3	Model	58.49	9	6.50	260.63	<0.0001	***
	X1	0.1275	1	0.1275	5.11	0.0582	*
	X2	4.80	1	4.80	192.69	<0.0001	***
	X3	50.15	1	50.15	2011.12	<0.0001	***
	X1X2	0.0056	1	0.0056	0.2256	0.6493
	X1X3	0.0324	1	0.0324	1.30	0.2918
	X2X3	1.27	1	1.27	50.75	0.0002	***
	X12	0.0960	1	0.0960	3.85	0.0905	*
	X22	0.4271	1	0.4271	17.13	0.0044	***
	X32	1.42	1	1.42	57.00	0.0001	***
	Residual	0.1746	7	0.0249
	Lack of fit	0.1151	3	0.0384	2.58	0.1911
	Pure error	0.0595	4	0.0149
	Cor total	58.67	16

Note: *** p < 0.01 (extremely significant); 0.01 ≤ ** p < 0.05 (significant); 0.05 ≤ * p < 0.1 (more significant); p > 0.1 (not significant).

. The variance analysis results of the pickup rate, breakage rate, and impurity rate are shown in Table 5. The regression equations of Y₁, Y₂, and Y₃ on X₁, X₂ and X₃ were established, and their significance was tested.

(1)

Establishment of the regression equation and significance analysis of the pickup rate

The variance analysis for the pickup rate indicated that in this regression model, X₂, X₃, X₁X₂, X₁X₃, and X₁² had an extremely significant impact on the pickup rate model. X₂X₃ had a more significant impact on the pickup rate model. The significance of the influence of each variable on the pickup rate was in the following order, from more to less significant: the speed of the strip brush roller shaft, the hole pitch height of the profiling rod, and the forward speed of the machine. After eliminating the insignificant factors, the quadratic regression equation of each variable on the pickup rate was obtained, as shown in Equation (29):

$$\begin{array}{l}{Y}_1=95.63+2.15X_2-0.6163X_3+0.9950X_1{X}_2\\ +1.10X_1{X}_3+0.450X_2{X}_3-2.67X_1{}^2-1.09X_3{}^2\end{array}$$

(29)

(2)

Establishment of the regression equation and significance analysis of the breakage rate

The variance analysis for the breakage rate indicated that the X₁, X₂, and X₂² had an extremely significant impact on the breakage rate model. X₁X₂ and X₃² had a significant impact on the breakage rate model. The significance of the influence of each variable on the breakage rate was in the following order, from more to less significant: the speed of the strip brush roller shaft, the forward speed of the machine, and the hole pitch height of the profiling rod. After eliminating the insignificant factors, the quadratic regression equation of each variable on the breakage rate was obtained as shown in Equation (30):

$$\begin{array}{l}{Y}_2=2.03-0.2375X_1+0.9575X_2-0.1450X_1{X}_2\\ +0.4800X_2{}^2+0.1550X_3{}^2\end{array}$$

(30)

(3)

Establishment and significance analysis of the impurity rate regression equation

The variance analysis for impurity rate indicated that X₂, X₃, X₂X₃, X₂², and X₃² had an extremely significant impact on the impurity content model. The significance of the influence of each variable on the impurity rate was in the following order, from more to less significant: the hole pitch height of the profiling rod, the speed of the strip brush roller shaft, and the forward speed of the machine. After eliminating the insignificant factors, the quadratic regression equation of each variable on the impurity rate was obtained as shown in Equation (31):

$$\begin{array}{l}{Y}_3=4.98-0.7750X_2-2.50X_3-0.5625X_2{X}_3\\ +0.3185X_2{}^2+0.5810X_3{}^2\end{array}$$

(31)

3.1. Response Surface Analysis

We used Design-Expert 11.0 software (Stat-Ease Inc., Minneapolis, MN, USA) to generate a response surface graph for the corresponding model, as shown in Figure 9. From the response surface analysis, the significant and relatively significant interactions among the forward speed of the machine X₁, the speed of the strip brush roller shaft X₂, and the hole pitch height of the profiling rod X₃ were obtained for the response value pickup rate Y₁, breakage rate Y₂, and impurity rate Y₃.

(1)

Influence analysis of the pickup rate.

Figure 9a–c shows the response surfaces of the interactions of X₁ and X₂, X₁ and X₃, and X₂ and X₃ to Y₁, respectively. As can be seen from Figure 9b, when the speed of the strip brush roller shaft was constant, the pickup rate increased first and then decreased with the increase in the forward speed of the machine. The forward speed of the machine was constant, and the pickup rate was positively correlated with the speed of the strip brush roller shaft. As can be seen from Figure 9c, when the hole pitch height of the profiling rod was fixed, the pickup rate increased first and then decreased with the increase in the forward speed of the machine. The forward speed of the machine was constant, and the pickup rate changed at first and then decreased gradually with the increase in the hole pitch height of the profiling rod. In Figure 9a, it can be seen that the hole pitch height of the profiling rod was constant. The pickup rate was positively correlated with the speed of the strip brush roller shaft. The speed of the strip brush roller shaft was constant, and the pickup rate showed a trend of first slowly increasing and then decreasing with the increase in the hole pitch height of the profiling rod.

The reason may be that, under the condition of the constant rotation speed of the strip brush roller shaft, as the forward speed of the machine gradually increased, the jujube and sand mixture after collecting the strips was easier to accumulate and form on the comb teeth. However, with the increasing forward speed, the accumulation speed of the jujubes and sand mixture was too fast, resulting in an increase in the force between materials, and the strip brush could not pick up the jujubes in time. Therefore, the accumulation state of the materials on the comb teeth became unstable, and the accumulated materials fell back and continued to spread and fall to both sides of the comb teeth, resulting in a decrease in the pickup rate. With the continuous increase in the hole pitch height of the profiling rod, the depth of the comb teeth entering the soil gradually became shallow, and the angle of the comb teeth entering the soil began to decrease. Due to the smaller angle of the comb teeth entering the soil, it was easier for the jujubes to climb along the comb teeth. At the same time, due to the shallower depth of the comb teeth entering the soil, the amount of sand accumulation on the inclined surface of the comb teeth began to decrease gradually so that more jujubes were able to be accumulated on the comb teeth and picked up. However, with the continuous increase in the hole pitch height of the profiling rod beginning constant, the amount of accumulated sand decreased in a large area. There was not enough sand support between the jujube and sand mixture, so the structure stacked between the jujubes was not sufficiently stable in the climbing process, resulting in loose falling and a decline in the pickup rate.

(2)

Analysis of the influence of the breakage rate

Figure 9d,e shows the response surface of Y₂ under the interaction of X₁X₂ and X₂X₃. In Figure 9e, when the forward speed of the machine was constant, the breakage rate increased with the increase in the speed of the strip brush roller shaft. When the speed of the strip brush roller shaft was 45–50 rpm, the change in the breakage rate tended to be stable with the increase in the forward speed of the machine. When the speed of the strip brush roller shaft was 50–65 rpm, the breakage rate gradually decreased with the increase in the forward speed of the machine. In Figure 9d, when the hole pitch height of the profiling rod was constant, the greater the speed of the strip brush roller shaft, the higher the breakage rate. When the speed of the strip brush rod shaft was constant, the breakage rate tended to be stable, and the influence of the hole pitch height of the profiling rod on the breakage rate was low.

The reason for this may be that the main force during the jujube-picking process was from the comb brush of the strip brush. With the increase in the speed of the strip brush roller shaft, the impact of the strip brush on the jujube continued to increase, resulting in an increase in the breakage rate of the jujube. When the speed of the strip brush roller shaft was 45–50 rpm, the damage caused by the strip brush to the jujubes was low due to the low rotating speed. With the increase in the forward speed of the machine, the accumulation speed of the jujube and sand mixture became faster, and the speed of the required strip brush also became faster. Therefore, in this stage, the change in breakage rate tended to be gentle. When the speed of the strip brush roller shaft was 50–65 rpm, the impact effect of the strip brush on the jujubes was obvious. As the forward speed of the machine increased, the stacking speed of the jujubes became larger, the impact of the strip brush became smaller, and the damage rate decreased.

(3)

Analysis of the influence of the impurity rate.

Figure 9g shows the response surface of Y₃ under the interaction of X₂X₃. The speed of the strip brush roller shaft was constant, and the impurity rate gradually decreased with the hole pitch height of the profiling rod. The hole pitch height of the profiling rod was constant, the speed of the strip brush roller shaft increased, and the impurity rate gradually increased.

The reason may be that as the hole pitch height of the profiling rod increased, the depth of the comb teeth entering the soil became shallow, and stones and other sundries in the jujube and sand mixture were not easily accumulated. When the hole pitch height of the profiling rod was constant, the speed of the strip brush roller shaft increased, and the impact effect of the strip brush became stronger. The action time of sundries on the pickup device and the conveying and separating device became shorter, and the sundries fell into the fruit collection box, resulting in an increase in the impurity rate.

3.2. Parameter Optimization and Test Verification

In order to achieve the best performance of the whole machine, the influencing factors in the prototype were optimized. According to the operating conditions, the performance requirements, and the above analysis results, the model was optimized and analyzed using Design-Expert 11.0 software. The constraint conditions were

$$\{\begin{array}{l}0.2\le X_1\le 0.4\\ 55\le X_2\le 75\\ 21\le X_3\le 121\\ 89\le Y_1\le 100\\ 0\le Y_2\le 4\\ 0\le Y_3\le 9\end{array}$$

(32)

The objective function was optimized and solved. The optimal parameter combination was as follows: the forward speed of the machine was 0.312m·s⁻¹, the speed of the strip brush roller shaft was 53.316 rpm, and the hole pitch height of the profiling rod was 59.974 mm. In order to verify the accuracy of the model, a verification test was conducted, as shown in Figure 10. We took the forward speed of the machine as 0.3 m·s⁻¹, the speed of the strip brush roller shaft as 53 rpm, and the hole pitch height of the profiling rod as 60 mm. We conducted five verification tests and took the average value. The results are shown in

Table 6. Test factors and levels.

Items	Y1/%	Y2/%	Y3/%
Model optimization value	94.72	1.92	3.91
Verification test value	92.11	2.07	4.15
Relative error/%	2.83	7.25	5.78

.

After verification, the error between the experimental value and the theoretical optimization value of the model was found to be less than 8%. The optimization model is thus reasonable and can satisfy the operation requirements.

3.3. Discussion

The pickup rate and impurity rate of the pneumatic jujube-collecting machine for dwarf and close cultivation systems developed by Zhang et al. [10] were 96.41% and 1.54%, respectively. The pickup rate and impurity rate of the pneumatic jujube harvester developed by Shi et al. [31] were 98.05% and 5.63%, respectively. The pickup rate for picking jujubes using a negative pressure airflow was significantly higher than that in this study. The main reason for this is that the method using a negative pressure airflow for picking is mainly applied to sticky soils with high moisture content, and it can be applied with good results to pick up jujubes from the ground in a flat environment. The cleaning effect is also relatively good. However, the method using negative pressure pickup has high requirements for the land environment, and its performance is not sufficiently stable.

The pickup rate of the air blow-type ground-jujube picking device developed by Pan et al. [11] was 85.7%. The new-type ground-jujube harvest machine developed by Lu et al. [12] had an average harvest rate of 93.6% and a damage rate of 2.1%. The pickup rate of the ground-jujube pickup device developed by Wang et al. [14] was 90.8%, and the impurity rate was 5.7%. In this study, the jujube pickup rate was 92.11%, the impurity rate was 4.15%, and the breakage rate was 2.07%. The jujube pickup rate was relatively high, and the impurity rate and breakage rate were relatively reduced. The main reason is that comb teeth can shovel jujubes in a low-lying area and then pick them up. The sandy soil mainly falls from the clearance of the comb teeth, and other impurities are removed by the rear fan, so the impurity rate is relatively low. The rationality of the structure and operation parameters is an important reason for the decline in the breakage rate.

In this study, the method of using comb teeth entering the soil and picking up the jujubes with a brush can cope with different land environments, and its versatility thus becomes stronger. The articulated form of the pickup device and the conveying and separating device make the structure of the whole machine more concise and the performance more stable. Additionally, the research group found that the size, shape, and moisture content of the jujube branches and leaves are key factors affecting the impurity rate. The maturity of the jujubes was also found to have a certain influence on the breakage rate.

4. Conclusions

In this paper, a mechanical floor-standing jujube picker for manual strip collection was designed according to the characteristics of the high fruit drop rate of jujubes in Xinjiang during the harvest period. The Box–Behnken method was used to evaluate the performance of the mechanical floor-standing jujube picker. The results showed that the pickup rate, breakage rate, and impurity rate were 94.72%, 1.92%, and 3.91%, respectively. As the forward speed of the machine was 0.312 m∙s⁻¹, the speed of the strip brush roller shaft was 53.316 rpm, the hole pitch height of the profiling rod was 59.974 mm, respectively. Furthermore, the field verification tests were conducted according to the optimal parameter combination conditions, of which results showed that the pickup rate, breakage rate, and impurity rate were 92.11%, 2.07%, and 4.15%, respectively. The mechanical floor-standing jujube picker’s operation performance meets the jujube harvesting requirements. The jujube pickup rate and impurity rate were significantly improved compared to the traditional mechanical jujube pickers, with simple structure and reliable performance. This research can provide a new mechanized operation method for picking up jujubes. Additionally, it provides a theoretical reference for the subsequent mechanized harvesting of jujubes.

In the future, this study can be further improved in two aspects. First, we should continue to improve the structure of the pickup device and optimize the operation parameters to improve the pickup rate. Secondly, future research should focus on reducing the impurity rate of the mechanical floor-standing jujube picker.