Design of Hydrostatic Power Shift Compound Drive System for Cotton Picker Experiment

Abstract

To meet the working performance demand of cotton pickers, a hydrostatic power shift composite drive system design is proposed. This study aims to enhance the driving function of the cotton picker in various working conditions and improve its adaptability by combining a hydrostatic speed control system with a mechanical power shift structure. To achieve this, a single variable pump + double variable motor closed circuit is adopted. By adjusting the pump and motor displacement in stages, the driving speed of the cotton picker can be optimized for different working conditions. Additionally, the power shift mechanism is employed to increase the speed range and improve the transmission efficiency, enabling higher speeds to be achieved. Firstly, the main components of the composite drive system were calculated and selected, and then AMESim software was used for modeling and simulation analysis, and the results are as follows: When the cotton picker starts and picking operation stage variable displacement pump + fixed displacement dual motor speed control, the highest driving speed is 8.5 km/h. During the field and road transport operation stage fixed displacement pump + variable displacement dual motor speed regulation, the highest speed of 14.5 km/h was achieved in the field. When transferring to the road, the instant mechanical power shift speed and, the highest speed on the road was up to 27.5 km/h. Finally, the field experiment and speed ratio analysis of the drive system was conducted, and the average error of the experimental speed measurement was 0.588%. The speed ratio matching was in line with the design expectation. The results show that the hydrostatic power shift composite drive system designed in this study has good driving adaptability and can effectively meet the functions of cotton picker field picking, transport operation and road transportation in transit, which provides theoretical support for the design of cotton picker chassis drive system.

1. Introduction

Cotton is a major cash crop in China, which is both an important economic source for Chinese farmers and related to the employment of people in textile and other industries [1,2]. Cotton mechanization harvesting is the key to the whole mechanization of cotton production, and cotton picking machine automation has become a development trend [3,4,5,6]. Cotton pickers operate under complex working conditions and need to be transported in the field after picking and packing, and, at the end of the field operation, transferred to the next cotton field to continue harvesting, which will be transported by road. In the field picking operation, there is the load fluctuation, the walking system output power is at low speed, and there is high torque; in the transport operation, the load is smaller, the walking speed is fast, walking power system output power high-speed, and low torque [7]. The cotton picker operation process requires the drive system to be able to change the speed and torque in real time to adapt to the continuous changes in the actual load under harsh working conditions [8,9,10]. An operating speed that is too fast tends to cause blockages, leading to the machine’s failure and even fires. If the operating speed is too slow, not only will the advantages of the machine’s cotton picking efficiency not be fully exploited, but also the cotton content rate will increase, resulting in poor harvesting results [11,12]. Therefore, a proper transmission is extremely important for the quality of the cotton picker’s work.

Hydrostatic power shift drive technology combines the speed control and fast response of hydrostatic transmission with the high efficiency and reliability of mechanical transmissions [9]. Compared with a single hydrostatic transmission, hydraulic transmission, and mechanical transmission, hydrostatic power shift drive has advantages of infinitely variable, hydrostatic speed regulation and mechanical power shift speed changes, and it has a large speed range in each corresponding gear, a simple structure layout, it is flexible and easy to control, and it is easy to realize speed regulation and load adaptation in a cotton field with complex road conditions and in harsh environments [13,14,15]. Therefore, the study of hydrostatic power shift compound drive is of great significance to the development of the chassis driving system of cotton pickers.

In recent years, the research and development of agricultural machinery and engineering equipment have gradually become the focus, especially design and experimental research on drive systems for agricultural tractors and large, non-road vehicles. Seung-Min Baek developed a simulation model of a 50 kW hydro mechanical transmission tractor and conducted a performance analysis, testing, and experimental evaluation. Young-Jun Park developed a tractor Hydraulic machinery transmission model, analyzed the power transmission characteristics of a tractor equipped with a Hydraulic machinery transmission, and confirmed that the power of the main shift can be split and circulated according to how the hydrostatic unit travels [16,17]. Kim, W.S., Kim, Y.S. et al. analyzed the required power of agricultural tractors with power outputs of 78 kW, 80 kW, and 120 kW and conducted on-site testing on the main power source shaft, power extraction device, main hydraulic pump, and auxiliary hydraulic pump [18,19,20,21]. Bi Xinsheng introduced the hydrostatic travel drive system applied to a self-propelled cotton picker and analyzed the main components, working principle, and dynamic characteristics of the hydrostatic travel drive system [22,23]. Ender İnce developed a new power distribution input coupled with an IVT system and studied the influence of various dynamic parameters, such as power flow and the Willis transmission ratio, on the mechanical efficiency of the system [24]. Chen Fa designed a hydraulic walking transmission scheme for self-propelled cotton pickers by analyzing the external characteristics of the engine and the speed regulation characteristics of the hydraulic system, which can achieve a continuous, variable transmission [25]. Ding Li designed the hydraulic travel drive system with an upland gap sprayer as the research object [26]. According to the agronomic characteristics of tobacco harvesting and the operational requirements of tobacco harvester, Chenhui Zhu proposed the hydraulic traveling drive scheme with two pumps and two motors, made the calculation and selection of power units and hydraulic components in the traveling drive system, and simulated and analyzed the hydraulic output characteristics of stopping, slope full start and flat differential steering conditions [27,28]. Ni Xiangdong’s team designed a hydraulic–mechanical, continuously variable transmission for the traveling system of a cotton picker (initial picking speed: 0–5.6 km/h; re-picking speed: 0–7.8 km/h; road transport: 0–25 km/h) in a harsh environment, with a variable load and a poorly matched driving power, and explored its transmission characteristics and shift quality through simulations and experiments based on PID optimization and BP. The transmission shift control strategy was developed based on PID optimization and the BP neural network to optimize its speed control characteristics, and the feasibility of the design was verified by carrying out an experiment on a prototype [29,30,31,32,33].

The traditional hydraulic–mechanical, stepless transmission drive speed is unable to meet the current needs of cotton picker operation driving; both domestic and foreign have used hydrostatic mechanical composite transmission as the mainstream of the cotton picker chassis transmission. In this paper, we take six rows of self-propelled cotton picker walking drive systems as the research object, design a hydrostatic power shift composite drive system through theoretical calculation selection, virtual simulation technology and experiment, and other methods of the design of the composite drive system for hydraulic system characteristics analysis and mounted cotton picker field experiment verification, in order to provide reference theoretical basis for the development of domestic cotton picker drive system and composite drive research.

2. Design and Selection of Compound Drive System for Cotton Picker

2.1. Analysis of Cotton Picker System and Driving Parameters

Cotton picker system and chassis drive composition as shown in Figure 1, cotton picker system mainly by the picking system, cab, electrical system, seed cotton conveying system, cotton collection system, lubrication system, baling system, power system, hydraulic system and transmission and walking system composition, chassis drive mainly by the engine, transfer case, hydrostatic transmission, power shift transmission (PST), front axle, rear axle, and wheel composition. Different cotton picker driving main parameters are shown in

Table 1. Main parameters of the drive systems of different cotton pickers.

Driving Parameters	Cotton Picker Model
	John Deere CP690	SSCIM 4MZD-6	CRCHI 4MZD-6
Engine rated power (kW)	418	410	298
Speed of initial mining in field (km/h)	7.1	7.1	6.8
Speed of picking cotton (km/h)	8.5	8.5	8
Speed of field transportation (km/h)	14.5	15	14.5
Speed of highway transportation (km/h)	27.4	27	27.4

CRCHI: China Railway Construction Heavy Industry Co., Ltd. SSCIM: Shandong Swan Cotton Industrial Machinery Co., Ltd. (Jinan, China).

; the driving speed ranges are about the same, with the picking speed going up to 8.5 km/h, and transport speed going up to 27.4 km/h.

2.2. Overall Design of Chassis Compound Drive System

Figure 2 shows the sketch of the compound drive system of a cotton picker, which is composed of hydrostatic transmission and power shift transmission. The power source of the system is the engine; the power is through the diverter box into the variable pump, and then through the hydraulic subsystem of each output to the front drive, rear drive motor, and finally the front drive motor hydraulic energy through the power shift transmission to drive the front axle, rear drive motor hydraulic energy directly to drive the rear axle. This system can be adjusted by switching the clutch (7) and brake (8) cotton picker transmission ratio and working mode.

The closed-volume speed control circuit of the cotton picker drive hydraulic system consists of a single variable pump and dual variable motors in parallel, as shown in Figure 3. To improve the variable torque range and transmission efficiency of the system, the two motor displacements form a certain difference, where the rear drive is a small displacement motor, and the front drive is a large displacement motor, and the full displacement of the front drive motor is 1.3~1.8 times larger than the full displacement of the rear drive motor.

The structure principle and power flow of the front axle PST of the cotton picker are shown in Figure 4, which mainly includes input sun wheel s, duplex planetary wheel p, clutch C, brake B, planetary frame j, and output internal gear ring q. The sun wheel s is connected to the power input shaft via a key and meshes with the large end gear of duplex planetary wheel p. The clutch friction plate is rigidly coupled with the power input shaft, the clutch hub is rigidly connected to the planetary frame, the brake friction plate end is solidly connected to the clutch hub, and the brake hub is connected to the frame. The brake friction plate is rigidly coupled with the power input shaft, the clutch hub is rigidly coupled with the planetary carrier, the brake friction plate is rigidly coupled with the clutch hub, the brake hub is rigidly coupled with the clutch hub to form a nested clutch and brake, and the brake hub with coupled to the frame. The planetary wheel is mounted on the planetary frame employing a bearing, and the output inner ring q is mounted on the power input shaft by means of a bearing and meshes with the small end gear of the duplex planetary wheel p.

The power shift transmission is controlled by the engagement/disengagement of the brake and clutch to obtain different operating states of the transmission and different operating modes of the cotton picker, according to the control logic shown in

Table 2. Working mode control logic.

Working Mode of Cotton Picker	Transmission Status	Brake	Clutch
--	Neutral	0	0
Field mode	Low-speed Gear	1	0
Highway mode	High-speed Gear	0	1
--	Locked	1	1

0: The friction plate is separated from the steel plate. 1: The friction plate is joined to the steel plate.

. When the cotton picker is in field operation mode, the brake is combined, the clutch is separated, and then the clutch friction plate rotates with the power input shaft, the clutch hub and the planetary frame are fixed to the frame, the transmission is in low gear, and the power flow is as shown in Figure 4a. When the cotton picker is in road mode, the brake is separated, and the clutch is engaged. At this time, the sun wheel S2, high-speed clutch and planetary wheel are rigidly connected with the power input shaft and rotate together with the power input shaft, and the transmission is in direct gear, i.e., high-speed gear, and the power flow is as shown in Figure 4b.

2.3. Overall Design of Chassis Compound Drive System

2.3.1. Cotton Picker Drive-Related Power Calculation

Traveling Power Calculation

Cotton picker walking power formula:

$$N_x=\frac{m_c{gv}_c{f}_r}{{\eta }_z}$$

(1)

where N_x is the walking power of the cotton picker, v_c is the driving speed of the cotton picker, km/h; η_z is the total efficiency of the engine to drive wheel transmission; f_r is the rolling resistance coefficient (f_r = 0.12), m_c is the mass of the cotton picker (m_c = 33,000 kg).

Climbing Power Calculation

Calculate the driving power required for the cotton picker to climb an 11° ramp:

$$N_x=\frac{F_k{v}_p}{3600}$$

(2)

where F_k is the total resistance of the driving process, N; v_p is the climbing speed (v_p = 2.5 km/h).

$$F_k=F_f+F_i+F_a$$

(3)

where F_f is the rolling resistance of the wheel, N; F_i is the slope resistance, N; F_a is the acceleration resistance, N. Among them,

$$F_f=G{f}_r=m_{c}g{f}_r$$

(4)

$$F_i=m_{c}g\sin \alpha$$

(5)

$$F_a=\left(m_c+{\delta }_m\right)\frac{d{v}_c}{dt}$$

(6)

where m_c is vehicle mass, kg; g is the acceleration of gravity, m/s²; f_r is the coefficient of wheel rolling resistance; α is the slope Angle, rad; δ_m is the equivalent translation mass, kg. The sliding power for driving in climbing:

$$N_{\delta }=\frac{F_k{v}_p\delta }{3600(1-\delta )}$$

(7)

where δ is the slip rate (δ = 0.05).

Climbing power is:

$$N_p=N_x{+N}_{\delta }$$

(8)

When the cotton picker reaches 2.5 km/h during speed climbing, calculated by Formula (8), climbing power can be obtained from the cotton picker crawling power of 80 kW.

Cotton Picker Angle Power

Angle power reflects the hydraulic walking transmission system power capacity and transformation capacity, which is the maximum torque of the driving wheel and the highest speed of the driving wheel product. The cotton picker walking transmission system torque and speed are converted to angular power single parameter matching, to obtain the angular power formula:

$$P_{jj}=\frac{M_{max}{n}_{max}}{9549}=\frac{F_{max}{v}_{max}}{3600}$$

(9)

where P_jj is the angular power of the cotton picker, kW; M_max is the maximum torque of the driving wheel, N·m; n_max is the maximum speed of the driving wheel, r/min; F_max is the maximum tangential traction force, N; v_max is the maximum theoretical speed, km/h.

2.3.2. Selection of Key Components

Engine Selection

The engine is the power source of the machine; the cotton picker operation power consumption mainly includes walking power, picking head drive power, packing power, fan power, climbing power, steering and action pump power.

Cotton picker packing power calculation formula is as follows:

$$N_d=\frac{Gg{v}_d{f}_d}{{\eta }_d}$$

(10)

where G is the mass of cotton bale (G = 2500 kg), v_d is the packing line speed (v_d = 1.6 m/s), η_d is the total efficiency of packing transmission (η_d = 0.85), f_d is the rolling resistance coefficient of packing (f_d = 0.7).

Cotton picker single fan power calculation formula is as follows:

$$N_f=\frac{Q_o{P}_f}{102\times 3600{\eta }_f{\eta }_{fd}}$$

(11)

where Q_o is the fan flow (Q_o = 15,000 m³/h), P_f is the fan full pressure (P_f = 6.8 kPa), η_f is the fan efficiency (η_f = 0.7), η_fd is the fan transmission efficiency, fan belt drive (η_fd = 0.95).The single picking head drive power is 11 kW [34], the cotton picker is equipped with six picking heads, the steering and action pump consumes 40 kW, the operational walking power is 80.5 kW calculated by Formula (2), the baling power is 32.2 kW calculated by Formula (10), the single fan power is 42.9 kW calculated by Formula (11), and the cotton picker double fan conveys cotton. The cotton picker for picking operations requires power for its consumption of maximum power conditions, taking the engine power reserve coefficient of 1.3; according to the above analysis calculations, the cotton picker starts with the minimum power requirements of 395.9 kW. The engine is selected as Dongfeng Cummins QSZ13-C550-30 diesel engine (Dongfeng Cummins Engine Co., Ltd., High-tech Industrial Development Zone, Xiangyang, China) with rated power of 410 kW and rated speed of 1900 r/min.

Variable Motor Selection

The cotton pickers drive a hydraulic transmission system by a single pump double motor to form a closed volume speed control circuit; the motor is the hydraulic walking transmission system executive components, and motor selection affects the dynamics and economy of the cotton picker. According to the angular power matching motor parameters, the angular power of the motor is determined as follows:

$$P_{mj}=\frac{P_{jj}}{Z{\eta }_2}$$

(12)

where P_mj is the motor angular power, kW; Z is the number of motors (Z = 2), and η₂ is the motor and drive wheel between the reducer transmission efficiency (η₂ = 0.80).

The cotton picker power shift transmission has two gears, so there are two angular powers; the maximum angular power is the basis for motor selection. According to Formula (12) can be calculated to meet the minimum specifications of the cotton picker motor.

Next, match the calculated motor displacement:

$$P_{mj}\le \frac{0.95\Delta P_m{V}_{mmax}{n}_{mmax}}{60000}$$

(13)

where ΔP_m is the highest matching pressure of the hydraulic travel system (ΔP_m = 45 MPa), V_mmax is the maximum displacement of the motor, mL/r; n_mmax is the maximum speed of the motor, r/min.

Combined with the hydraulic motor selection principles, two Linde hydraulic HMV-02 series swashplate variable motors are selected, and their main technical parameters are shown in

Table 3. Variable motor and pump model parameters.

Parameters	Front Drive Motor	Rear Drive Motor	Variable Displacement Pump
Maximum Displacement/mL∙r−1	165	105	105
Maximum working pressure/MPa	45	45	45
Continuous work stress/MPa	25	25	25
Maximum operating speed/r∙min−1	3900	4700	4700
Continuous working power/kW	214	153	153

.

Variable Pump Selection

The power source of the cotton picker is distributed by the splitter box to provide power for the variable pump of the hydraulic drive system. Through the gear ratio of the transfer case, select the appropriate variable pump. Engine, transfer case and variable pump speed matching relationship is as follows:

$$n_{pH}{=n}_{eH}·i_1$$

(14)

where n_pH is the variable pump-rated speed, r/min; n_eH is the engine-rated speed, r/min; i₁ is the transfer case transmission ratio.

In the hydraulic travel transmission system of the cotton picker, the variable pump is matched with the following relationship:

$$V_{pmax}\ge \frac{V_{mmax}{n}^s{}_{mm}}{{\eta }_m{}_v{\eta }_{pv}{n}^s{}_{pH}}$$

(15)

where V_pmax is the variable pump maximum displacement, mL/r; V_mmax is the variable motor maximum displacement; n_smm is the maximum speed at the maximum displacement of the motor; n_spH is the rated matching speed of the pump.

Take the variable pump and variable motor volumetric efficiency of 0.95, matching the calculation of V_pmax ≥ 207.4. The transmission ratio of the transfer case is i₁ = 1.2. Combined with the variable pump selection principles, the choice of Linde hydraulic HPV-02 variable pump, the main technical parameters are shown in Table 3.

3. Simulation and Experimentation

3.1. Modeling of Compound Drive System of Cotton Picker

AMESim in the field of mechanical and hydraulic modeling has obvious advantages; the module library retrieves the appropriate modules and components, without building mathematical equations and transfer functions, can build the corresponding system model, and set parameters to complete the system modeling and simulation.

The overall model of the compound drive system of the cotton picker is shown in Figure 5, which mainly includes the engine model, single pump and double motor closed volume hydraulic circuit model, power shift transmission model, and cotton picker load model, among which the pump motor variable displacement model is shown in Figure 6a,b and the power shift clutch control valve model is shown in Figure 7.

3.2. Simulation Setup and Experiment Methods

3.2.1. Simulation Setup

The main parameters of the compound drive system of the cotton picker, such as variable pump, variable motor, closed volume circuit oil pressure, and weight of cotton picker, are shown in

Table 4. Simulation model parameter.

Component Name	Numerical Value and Unit	Component Name	Numerical Value and Unit
Engine speed	1900 r/min	Load viscous damping factor	0.5
Displacement of the pump	210 mL/r	Displacement of charge pump	38 mL/r
Power shift transmission ratio	2.38	Wheel side reduction ratio	16
Displacement of the front motor Picker mass	165 mL/r	Displacement of rear motor	105 mL/r
	33,000 kg	Full load mass	37,000 kg
Safety valve opening pressure	42 MPa	Flushing relief valve opening pressure	1.4 MPa
Charge relief valve opening pressure	2 MPa	One-way valve opening pressure	0.01 MPa
Front wheel radius	0.53 m	Rear wheel radius	0.43 m
Engine speed	1900 r/min	Load viscous damping factor	0.5
Displacement of the pump	210 mL/r	Displacement of charge pump	38 mL/r
Power shift transmission ratio	2.38	Wheel side reduction ratio	16

. The simulation time is set to 60 s, simulation step size is 0.01 s, the simulation of the cotton picker from the field operation to the road transport driving process is as follows: 0–25 s stage for the cotton picker field start to field transport speed process, including the first picking, re-picking, and transport three conditions, because the picking process transmission principle is the same and picking for the continuation of the first picking, here ignoring the discussion of the first picking stage. Among them, 0–10 s is set as the speed regulation process of the picking process, 10–25 s is the speed regulation process of field transportation, the transmission shift action is set to 25 s, and 25–60 s represents the road transportation stage.

The hydraulic system pump, motor displacement ratio, and clutch control signal as shown in Figure 8, 0–25 s. The cotton picker field operation hydrostatic speed regulation, transmission no shift action, 0–10 s front drive, a rear drive motor for full displacement, pump displacement from 0 gradually increased to the maximum; 10–25 s variable pump to maintain full displacement, the system flow rate remains unchanged, front drive, rear drive motor displacement ratio from 1 to 0.55, 0.60, respectively, 0.60. From 25–60 s, 25 s when the front drive, rear drive motor displacement due to the transmission shift action, to keep the instantaneous speed unchanged, the front drive, rear drive motor displacement ratio transient increase to 0.81, to adjust the speed to the highest speed of road driving, front drive, rear drive motor displacement ratio again from 0.81 to 0.54, 0.56, respectively.

3.2.2. Experiment Methods

After the whole cotton picker prototype parts are tested and assembled, the cotton picker drive system mainly includes the front axle drive motor, rear axle drive motor, front axle power shift transmission, differential, parking brake, etc. Figure 9a–e shows the assembly of the chassis drive module part of the cotton picker. The front drive motor is bolted to the gearbox and can be removed and replaced, and the variable motor is connected to the powershift transmission. The transmission is bolted to the chassis of the cotton picker and the power output shaft is connected to the front axle drive.

As shown in Figure 9f–i, the cotton picker real vehicle experiment was conducted according to the Chinese standard “Cotton Harvester” (GB/T 21397-2008) design experiment. The cotton picker road speed experiment schematic is shown in Figure 10. The cotton picker is recorded during field primary picking, field re-picking, field transport, and road transport work mode, three times through the stable speed measurement zone 20 m time.

Operating speed performance measurement: before and after the experiment area, with a 20 m stability zone, the cotton picker functions according to the normal operating speed for harvesting, operating speed to maintain consistency, to determine the time of the cotton picker through the 20 m measurement zone. The cotton picker speed calculation formula is:

$$v=3.6\frac{L}{\Delta t}=3.6\frac{L}{t_1-t_0}$$

(16)

where v is the cotton picker operating speed, km/h; L is the length of the cotton picker measurement area, m; ∆t is the cotton picker through the measurement area time, s; t₀ is the corresponding time of the test starting point, t₁ is the corresponding time of the end-point.

4. Results and Discussion

4.1. Simulation Results and Analysis

The travel speed of the cotton picker is shown in Figure 11. The speed reaches the maximum speed of 8.50 km/h for re-picking at 10 s, 15 km/h for field transport at 25 s, and 27.05 km/h for road transport at 50 s. At 25 s, the transmission clutch executes a shifting action, and due to the existence of the shifting time, the speed drops instantaneously, and shifting is completed at 25.3 s. At the same time, due to the rapid changes in the shifting ratio and motor displacement, the speed of the vehicle undergoes a transient sudden change, i.e., the speed increases to 18.61 km/h at 26.2 s. The speed regulation process of the cotton picker road transport is 25–60 s; the speed is controlled by the front drive and rear drive motor to adjust the displacement together, the displacement remains constant at 50 s, and the speed reaches the maximum transport speed.

The hydraulic system motor output speed is shown in Figure 12, where 0–10 s is the pump variable displacement speed regulation, the system flow increases with the pump displacement, and the motor output speed change trend is consistent with the pump displacement change. The front drive, and rear drive motor variable displacement speed regulation is 10–25 s, and the motor output speed is inversely proportional to the displacement. In addition, the 25–25.3 s process is the speed fluctuation caused by the mechanical shift, indicating that the hydraulic effect is larger at this moment. The 25–60 s process is the same as the previous stage, the motor output speed trend is inversely proportional, with the highest speed moment, the front drive motor stable speed reached 2627.70 r/min, and the rear drive motor speed reached 1479.45 r/min.

The hydraulic system oil pressure is shown in Figure 13. As can be seen in the figure, the system oil pressure fluctuates during the cotton picker starting phase, the highest oil pressure is close to 16 MPa; during the variable pump speed regulation phase, the oil pressure is stabilized at 10.08 MPa, the cotton picker reaches the highest speed of 8.5 km/h during the first picking task, and the system’s high-pressure oil pressure drops from 10.18 MPa to 5.91 MPa. During the acceleration of the cotton picker’s field transport, the system is regulated by the front drive and rear drive variable motors. The oil pressure rises to 16 MPa, and the oil pressure decreases, and finally stabilizes at 9.22 MPa when the cotton picker reaches the maximum speed of 14.5 km/h during field transport. During the acceleration of the cotton picker during road transportation, the oil pressure fluctuates greatly at the power shift point and gradually increases to 19.73 MPa at the stage when variable motor displacement ratio is reached, and then decreases again. In addition, when the speed of the cotton picker road transportation is stabilized at 27.5 km/h, the oil pressure stabilizes at 12.83 MPa, and the oil pressure of the low-pressure oil circuit is 3.32 MPa.

The torque of the hydraulic system motor is shown in Figure 14. The torque fluctuates in the starting stage, as the speed increases to 8.5 km/h, while the torque of the front drive and rear drive motors decreases from 190.10 N·m to 82.29 N·m and 121.00 N·m to 52.37 N·m, respectively. In field transportation conditions, the variable motor regulates the speed and the torque of the front drive motor reaches a maximum of 176.12 N·m and stabilizes at 80.70 N·m, while the rear drive motor reaches a maximum of 122.26 N·m and stabilizes at 56.02 N·m. During the road transport phase, the torque gradually stabilizes as the speed steadily increases, with the maximum torque approaching 250 N·m for the front drive motor and 164 N·m for the rear drive motor. At the power shift point, the peak torque reaches 976.32 N·m for the front drive motor and 624.42 N·m for the rear drive motor. After 50 s, the speed reaches its maximum and remains constant for transport, as no acceleration exists and the motor torque plummets to a constant value.

In a hydraulic system flow as shown in Figure 15, the variable pump flow is equal to the sum of the two motors’ flow, 0–10 s process flow with the pump displacement change is increasing linearly, 10 s after the variable pump to maintain the maximum flow; at the 10–25 s and 25–60 s stages, motor flow with its own displacement and speed influence a small change, whereas at 25 s, due to mechanical shift, motor flow with speed fluctuations appears to suddenly change.

4.2. Experimental Results and Analysis

4.2.1. Experimental Results

The results of the walking speed experiment are shown in

Table 5. Table showing cotton pickers’ ability to pass the stability experiment interval.

Working Mode	Field Picking	Field Re-Picking	Field Transport	Road Transport
∆t1/s	10.32	8.51	5.08	2.68
∆t2/s	10.42	8.58	5.10	2.56
∆t3/s	10.16	8.56	4.82	2.66
Experiment speed/km∙h−1 Theoretical speed/km∙h−1	6.99	8.42	14.40	27.34
	7.00	8.50	14.50	27.50
Speed relative error	0.14%	0.94%	0.69%	0.58%

. According to the analysis of experimental data, the walking speed of the cotton picker meets the basic design requirements with an average relative error of 0.588%, under the working modes of initial picking in the field, re-picking in the field, field transportation, and road transportation. In addition, the harvesting effect in the field is good, as shown in Figure 16, which verifies that the design of the power shift drive system of the cotton picker is reasonable, and the system is feasible.

4.2.2. Speed Ratio Analysis

By controlling the operating handle of the cotton picker to change the walking speed, the variable displacement ratio of the pump, the variable displacement ratio of the front drive motor, and the variable displacement ratio of the rear drive motor in working modes during field picking, field transport, and road transport were collected to fit the speed ratio characteristic curve of the cotton picker, and then analyzed.

As shown in Figure 17, the driving speed of the cotton picker is divided into a low-speed section and a high-speed section. The low-speed section of the cotton picker includes the initial picking and re-picking mode in the field and the transport mode in the field. When the cotton picker works in field picking mode, the front drive variable motor displacement ratio and the rear drive variable motor displacement ratio are both 1, which is equivalent to the quantitative motor. By controlling the displacement ratio of the variable pump from 0 to 1, the driving speed of the cotton picker during the field picking increases to a maximum of 8.5 km/h. In field transport mode, the variable pump reaches a maximum displacement ratio of 1, which is equivalent to a dosing pump. Then, gradually reducing the front drive motor displacement ratio and rear drive variable motor displacement ratio can increase the driving speed to the maximum speed of field transport at 14.5 km/h. The cotton picker switches from low-speed section to high-speed section driving for road transport mode, and the maximum speed reaches 27.5 km/h. The results and the above analysis show that the design of the compound drive system of the cotton picker meets the actual operational requirements.

5. Conclusions

This study proposes a design of a hydrostatic power shift composite drive system for cotton pickers through the combination of a hydrostatic speed control system and mechanical power shift structure, using a single variable pump + double variable motor closed circuit. Adjusting the pump motor displacement to adjust the driving speed of the cotton picker and obtain different working conditions by segmentation, combined with the power shift mechanism to increase the speed range to obtain higher speed, realizes the different operating conditions of the cotton picker’s driving function.

The paper first calculates and selects the main components of the composite drive system, and then uses AMESim software to model and simulate the analysis. The simulation results are as follows: the maximum driving speed of the cotton picker is 8.5 km/h at the stage of low-speed high-torque starting and cotton picking operation, with variable displacement pump + fixed displacement dual motor speed regulation; the maximum driving speed is 14 km/h at the stage of high-speed low-torque transportation operation in the field and road transportation, with fixed displacement pump + variable displacement dual motor speed regulation. During motor speed regulation, the highest speed is 14.5 km/h, while in the field to the road transfer, when the mechanical power shift speed increase, the highest speed is 27.5 km/h.

Finally, in the field experiment and analysis, the actual speed experiment results between theoretical error is 0.588%, and the harvesting effect is good, meeting the actual operating speed requirements of the cotton picker. At the same time, the analysis of the speed ratio curve shows that the operating travel speed under each working condition is basically consistent with the theoretical requirements. The results show that the hydrostatic power shift compound drive system designed in this study has good driving adaptability and can effectively meet the functions of cotton picker field picking, transportation operation, and road transportation in transit, which provides theoretical support for the design of this cotton picker chassis drive system.