Serial hybrid tractor gear shifting mechanism and gear shifting control method

Abstract

The present application relates to a kind of serial hybrid tractor gear shifting mechanism and gear shifting control method, belong to hybrid tractor gear shifting technical field, the gear shifting mechanism of the present application only uses a set of electric push rod to cooperate section gear shifting slide and fork to realize three-position gear shifting, structure is extremely simple, high reliability, and with the function of power-off mechanical self-locking;On the basis of the gear shifting mechanism, combined with motor section closed-loop speed regulation, tractor zero to highest speed full range seamless stepless speed change can be realized, with mechanical transmission high efficiency and electric drive stepless smoothness, in addition, the present application also proposes generator gear shifting stable voltage pretreatment+driving motor timing compensation control strategy, can effectively alleviate the speed drop, the problem of jerk in the process of serial hybrid tractor gear shifting, reduce the traction force drop.

Description

Technical Field

[0001] This invention relates to the field of hybrid tractor shifting technology, specifically to a single electric push rod shift fork synchronizer shifting mechanism for a series hybrid tractor and its dedicated control method. Background Technology

[0002] The series hybrid tractor's transmission architecture uses an engine to drive a generator to produce electricity, which then powers the drive motor. A two-speed gearbox differentiates between low-speed field operations and high-speed road transport. Currently, mainstream gearboxing solutions in the industry suffer from the following drawbacks.

[0003] First, the hydraulic cylinder shifting mechanism requires a complete set of hydraulic components, including an oil pump, valve body, oil pipes, and accumulator. Tractors operate in harsh environments, and the hydraulic system is complex with numerous interfaces, making it prone to oil leaks. In winter, especially in Northeast China, where temperatures are extremely low, the hydraulic oil viscosity increases, easily leading to shifting delays and insufficient thrust. At the same time, hydraulic components occupy a large amount of space in the gearbox, resulting in high maintenance costs.

[0004] Secondly, the multi-actuator AMT shifting mechanism has multiple sets of push rods for independent gear selection and shifting, resulting in numerous parts, complex wiring harnesses, complex oil circuits, and large size; the multi-motor collaborative control logic is complex, and the electrical control cost is high; it is prone to pressure leakage and oil leakage, resulting in high maintenance costs in the later stage.

[0005] Third, existing two-speed hybrid tractors lack true continuously variable transmission (CVT) capability. Currently, the two-speed gearboxes in series hybrid tractors can only achieve fixed speed ratio switching, resulting in noticeable speed steps, shift jerks, and traction termination issues, making continuous stepless driving impossible. Relying solely on motor speed regulation presents the contradiction of high current heating at low speeds and poor efficiency at high speeds, failing to cover the optimal efficiency range across all operating conditions. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a shifting mechanism and shifting control method for a series hybrid tractor. It achieves three-position shifting using only one set of electric push rods in a simplified shifting mechanism. Combined with segmented closed-loop speed regulation of the motor, it realizes seamless stepless speed change of the tractor from zero to maximum speed, combining the high efficiency of mechanical transmission with the stepless smoothness of electric drive.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A shifting mechanism for a series hybrid tractor, housed within a gearbox housing, includes a power input shaft and a power output shaft. The power input shaft is connected to a drive motor via a spline. A working gear drive gear and a moving gear drive gear are fixedly connected to the power input shaft. A working gear driven gear and a transport gear driven gear are rotatably connected to the power output shaft. A synchronizer is mounted on the power output shaft between the two driven gears via a spline connection. The shifting mechanism also includes an electric push rod, a shift fork, and a segmented shift slide rod. The electric push rod is driven by a servo motor and connected to... The vehicle control unit (VCU) is connected for communication. The sectional shift slide is mounted on the gearbox housing. One end of the shift fork is fixed to the sectional shift slide by the shift fork locking nut, and the other end is connected to the synchronizer. The push block at the output end of the electric push rod is connected to the sectional shift slide on the outside of the shift fork. The electric push rod has three mechanical limit strokes: neutral, working, and transport. When shifting gears, the electric push rod drives the shift fork and the sectional shift slide to move synchronously, driving the synchronizer to slide axially until it engages with the driven gear of the working gear or the driven gear of the transport gear, thereby realizing gear switching.

[0008] Furthermore, the electric actuator has a built-in angle sensor, which can identify the work position of neutral, working, and transport modes across the entire range.

[0009] Furthermore, the section shift slide rod has three hemispherical limiting grooves along the axial direction, corresponding to the working gear, neutral gear, and transport gear positions respectively. The positioning ball of the slide rod is pressed into the limiting groove by the positioning spring installed on the gearbox housing, so as to realize the independent rigid locking of the three gear positions.

[0010] Furthermore, the electric actuator outputs linear motion through a ball screw thread pair and achieves self-locking through a ball screw self-locking nut.

[0011] Furthermore, the power input shaft is a hollow shaft, and a PTO drive shaft passes through the inside of the power input shaft. The PTO drive shaft is directly connected to the engine, and the reverse gear of the hybrid tractor is achieved by reversing the drive motor.

[0012] A shift control method for a series hybrid tractor, the shift control method being implemented based on the shift mechanism, includes the following steps: S0, stepless speed regulation zone prediction; VCU automatically determines the current optimal speed regulation zone based on the driver's throttle intention, real-time load, and motor speed: low speed heavy load automatically enters the working speed regulation zone, and high speed light load automatically enters the transport speed regulation zone. S1. Hybrid shift voltage regulation preprocessing: Before shifting, the VCU sends a command to the generator controller to increase the generator output power to stabilize the DC bus voltage and avoid bus voltage drop and power fluctuation during the shifting process. S2, Active torque reduction and synchronous disengagement: The VCU sends a torque unloading command to the drive motor controller to reduce the output torque of the drive motor; then it sends a command to the electric actuator to push it back to the neutral position with a light push force. S3, graded thrust engagement; the VCU controls the electric actuator to operate with three levels of variable thrust according to the target gear position: the first level of light thrust is used in the neutral shifting stage; the second level of medium thrust is used for flexible engagement in the synchronous engagement stage; and the third level of large thrust is used to ensure complete engagement in the gear position stage. The three levels of thrust are achieved by the VCU controlling the output torque of the servo motor in segments according to the real-time feedback of the actuator position from the angle sensor. S4, Gear shifting + dual closed-loop stepless connection; After the gear is in position, the VCU adopts cascade dual closed-loop speed regulation control of vehicle speed outer loop and drive motor torque inner loop. The working gear and the transport gear are configured with independent PID parameter groups respectively. The two speed ratio overlap ranges are seamlessly connected through the smooth interpolation algorithm of the motor target speed. S5, Hybrid torque compensation; Before shifting gears, the VCU pre-stores the target traction force at the wheel end. After disengaging the gear, the drive motor switches the speed closed-loop and pre-adjusts to the synchronous speed of the target gear. During the shifting process, the bus power is kept stable. After the gear is engaged, the drive motor quickly outputs the preset torque to restore the traction force. After the gear is fully stable, the system exits the shift compensation logic and returns to the dual closed-loop speed control corresponding to the gear. S6. Power-off self-locking anti-disengagement: After the gear position is stable, the VCU sends a power-off command to the servo motor, relying on the self-locking characteristics of the ball screw thread pair of the electric actuator to prevent disengagement.

[0013] Furthermore, in step S2, when a top tooth stall fault is detected, the push rod small reciprocating fine adjustment + motor micro speed disturbance fault tolerance logic is automatically executed, and the output drives the motor micro speed disturbance to finely adjust the gear phase until the synchronizer sleeve and the engaging tooth are smoothly aligned and meshed.

[0014] Furthermore, the dual closed-loop speed control specifically involves the outer loop receiving the driver's target vehicle speed and the actual vehicle speed, and outputting the target setpoint value for the motor through PID calculation; the inner loop of the drive motor receives the setpoint command output by the outer loop, and combines the actual motor speed and torque feedback to achieve rapid adjustment of the motor.

[0015] Furthermore, in the dual closed-loop speed control, the low-speed, high-torque PID parameters are used in the working gear range, which is suitable for plowing, deep plowing, and heavy-load operations; while the medium-speed, high-efficiency PID parameters are used in the transport gear range, which is suitable for transshipment and light-load operations.

[0016] Furthermore, in step S4, in the overlapping area between the highest speed of the working gear and the lowest speed of the transport gear, the VCU performs linear or S-shaped interpolation transition on the target speed of the drive motor, so that the whole vehicle has no speed step and no jerking.

[0017] Beneficial effects: 1. The shifting mechanism has an extremely simple structure and extremely high reliability; all hydraulic shifting systems and multiple electric actuators are eliminated, and a single electric push rod completes all shifting actions. The number of parts in the shifting actuator is reduced, and the maintenance difficulty and cost of the whole machine are significantly reduced.

[0018] 2. Power-off mechanical self-locking. Unlike conventional solutions that rely on electromagnetic brakes or motors to maintain torque, this invention utilizes the self-locking nut of the electric actuator's rolling screw, the positioning ball of the slide rod, and the limiting groove on the section shift slide rod to form a power-off mechanical self-locking structure, achieving double pure mechanical self-locking. The servo motor can be completely powered off, with no heat generation, no energy consumption, and high reliability, making it suitable for tractor plowing, rotary tillage impact loads, and bumpy road conditions.

[0019] 3. Achieve low-cost, mechanical, continuously variable transmission (CVT). Overcoming the speed step-by-step defects of traditional two-speed gearboxes, a combination of mechanical segmented speed ratios, dual closed-loop coupling speed regulation by the motor, and speed interpolation algorithms is used to achieve continuously variable smoothness comparable to high-end HSTs, while retaining the high efficiency of gear transmissions.

[0020] 4. Suppressing shift speed drop and jerking, reducing traction drop. This invention proposes a complete control strategy of generator shift voltage stabilization preprocessing + drive motor timing compensation: Before the shift action is executed, the vehicle controller controls the generator to adjust the output power in advance, stabilize the DC bus voltage, suppress the bus voltage fluctuation caused by the sudden change in drive motor power during the shift process, and provide the power basis for the motor's rapid response; during the shift window, the drive motor speed is pre-synchronized under the condition that the drive motor and the wheels are mechanically disconnected, and the speed matching of the new gear is completed in advance; when the synchronizer is detected to be engaged and the mechanical transmission path is restored, the drive motor quickly rebuilds the target drive torque, shortens the duration of power interruption, and effectively alleviates the speed drop and jerking problem during the shift process of the series hybrid tractor. Attached Figure Description

[0021] Figure 1 This is a cross-sectional schematic diagram of the two-speed structure of the tractor of the present invention.

[0022] Figure 2 This is a schematic diagram of the single electric push rod two-speed shifting mechanism of the present invention (the gearbox housing is not shown).

[0023] Figure 3 This is a partial sectional view of the section shift lever (showing the gearbox housing).

[0024] Figure 4 This is a flowchart of the shift control of the present invention.

[0025] Reference numerals in the attached diagram: 1. Power input shaft; 2. Working gear drive gear; 3. Transport gear drive gear; 4. Power output shaft; 5. Working gear driven gear; 6. Synchronizer; 7. Transport gear driven gear; 8. Electric actuator; 9. Section shift slide rod; 10. Slide rod positioning ball; 11. Speed ​​sensor; 12. Shift fork; 13. Push block; 14. Shift fork locking bolt; 15. Angle sensor; 16. Shift fork locking nut; 17. Pressure bolt; 18. Positioning spring. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0027] like Figure 1-3 As shown, this embodiment provides a single electric pushrod two-speed shifting mechanism for a series hybrid tractor with full-range closed-loop stepless speed regulation. This shifting mechanism is located within the gearbox and includes a power input shaft 1, a power output shaft 4, a synchronizer 6, an electric pushrod 8, and a shift fork 12. The series hybrid tractor includes an engine electronic control unit (ECU) and a vehicle control unit (VCU). The VCU communicates with the electric pushrod 8, the drive motor controller, and the engine ECU, respectively. The powertrain of the series hybrid tractor of this invention is: engine → generator → DC bus → drive motor → two-speed gearbox → wheels. The gearbox has only one set of electric pushrod assembly as the sole shifting actuator.

[0028] The power input shaft 1 and power output shaft 4 are rotatably connected to the gearbox housing. A working gear drive gear 2 and a transport gear drive gear 3 are sequentially fixed on the power input shaft 1. The power input shaft 1 is a hollow shaft connected to the drive motor via a spline, with a PTO drive shaft passing through it, which is directly connected to the engine. A working gear driven gear 5, a synchronizer 6, and a transport gear driven gear 7 are sequentially mounted on the power output shaft 4. The working gear driven gear 5 and the transport gear driven gear 7 are rotatably connected to the power output shaft 4. The synchronizer 6 is splined to the power output shaft 4 and can slide to engage with either the working gear driven gear 5 or the transport gear driven gear 7 under the action of the electric push rod 8 driving the shift fork 12. The working gear drive gear 2 and the working gear driven gear 5 mesh to form the working gear (low speed gear), and the transport gear drive gear 3 and the transport gear driven gear 7 mesh to form the transport gear (high speed gear). The reverse gear of the hybrid tractor of this invention is achieved by reversing the drive motor.

[0029] The electric actuator 8 is driven by a servo motor and communicates with the VCU. The section shift slide 9 is mounted on the gearbox housing and can move axially. One end of the shift fork 12 is fitted onto the section shift slide 9 and fixed to it by the shift fork locking bolt 14 and the shift fork locking nut 16. The other end of the shift fork 12 is connected to the synchronizer 6, which is a locking ring type synchronizer 6. The electric actuator 8 outputs linear motion through a ball screw thread pair. The output end of the electric actuator 8 is connected to a push block 13, which is U-shaped. The U-shaped push block 13 is connected to the section shift slide 9 outside the shift fork 12. When the electric actuator 8 drives the push block 13 to move, the push block 13 drives the shift fork 12 and the section shift slide 9 to move synchronously in a linear fashion along the axial direction of the section shift slide 9.

[0030] Furthermore, the section shift slide 9 has three hemispherical limiting grooves along the axial direction on its body, corresponding to the working gear, neutral gear, and transport gear positions respectively. When the section shift slide 9 moves to the target gear position with the shift fork 12, the corresponding limiting groove on the section shift slide 9 aligns with the slide rod positioning ball 10. The positioning spring 18 installed on the gearbox housing presses the slide rod positioning ball 10 into the limiting groove, realizing independent rigid locking of the three gear positions. The upper end of the positioning spring 18 is limited by the clamping bolt 17 installed on the gearbox housing.

[0031] The electric actuator 8 has three mechanical limit strokes: the middle position is neutral, the forward position is working, and the rear position is transport. The electric actuator 8 has a built-in angle sensor 15, which can identify the positions of neutral, working, and transport through a single sensor. The three-position mechanical limit is achieved by the segment shift slide rod 9, the slide rod positioning ball 10, and the shift fork locking bolt 14.

[0032] The shift fork 12 is fixed to the section shift slide 9 by the shift fork locking bolt 14. During shifting, the shift fork 12 is pushed back and forth by the electric push rod 8. Under the action of the shift fork locking bolt 14, the section shift slide 9 moves synchronously along the axial direction. The rolling screw self-locking nut of the electric push rod 8, the slide positioning ball 10, and the limiting groove on the section shift slide 9 together constitute a power-off mechanical self-locking structure, ensuring that the slide does not disengage under power loss conditions such as slopes and bumpy roads. When the section shift slide 9 moves to the target gear with the shift fork 12, the corresponding limiting groove on the section shift slide 9 aligns with the slide positioning ball 10. The slide positioning ball 10 is pressed into the limiting groove under the preload of the positioning spring 18, locking the section shift slide 9 axially and restricting the axial movement of the section shift slide 9 and the shift fork 12, thus completing the mechanical power-off self-locking.

[0033] The series hybrid tractor of the present invention has a speed sensor 11 near the power output shaft 4, and a built-in stepless speed regulation control architecture for the gear motor in the VCU: adopts a dual closed-loop coupling speed regulation of the motor, with an outer loop PID closed loop for vehicle speed and an inner loop PID closed loop for drive motor speed / torque; and achieves seamless stepless speed regulation across the entire vehicle speed range through segmented two-speed mechanical speed division and continuous speed regulation coupling of the drive motor.

[0034] like Figure 4 As shown, based on the above-mentioned single electric push rod two-speed shifting mechanism, the present invention also provides a single electric push rod control method for a series hybrid tractor with full-range stepless speed regulation, including: hybrid voltage stabilization shifting strategy + graded thrust anti-tooth-damping strategy + top tooth fault-tolerant micro-adjustment strategy + gear-motor dual closed-loop full-range stepless strategy, specifically including the following steps S0-S6.

[0035] S0, Stepless Speed ​​Regulation Zone Prediction. The VCU automatically determines the optimal speed regulation zone based on the driver's throttle input, real-time load, and motor speed: low speed and high load automatically enter the working gear stepless speed regulation zone; high speed and light load automatically enter the transport gear speed regulation zone.

[0036] S1. Hybrid shift voltage stabilization preprocessing. Before shifting, the VCU sends a command to the generator controller to increase the generator output power to stabilize the DC bus voltage and avoid bus voltage drop and power fluctuation caused by sudden changes in drive motor torque during the shifting process.

[0037] S2 and VCU send a torque unloading command to the drive motor controller to reduce the output torque of the drive motor; then send a command to the electric push rod 8 to push it back to the neutral position with a first-level thrust (light thrust); if the angle sensor 15 detects that the displacement of the push block 13 is not as expected (top tooth stall), execute the push rod small reciprocating fine adjustment + motor speed micro-matching fault-tolerant logic (output a small speed disturbance of the drive motor, fine adjust the gear phase, so that the synchronizer sleeve and the engagement tooth complete the tooth alignment), until the synchronizer 6 sleeve and the engagement tooth are aligned.

[0038] S3. Staged Thrust Gear Engagement. The VCU controls the electric actuator 8 to operate with three levels of thrust according to the target gear position: Level 1 thrust (light thrust to avoid energy loss during idle travel) is used during the neutral movement stage; Level 2 thrust (medium thrust for smooth gear engagement to avoid hard impact on the synchronizer) is used during the synchronization engagement stage; and Level 3 thrust (high thrust to ensure complete locking of the synchronizer sleeve and engagement teeth) is used during the gear engagement stage. The three levels of thrust are achieved by the VCU controlling the output torque of the servo motor in stages based on the real-time feedback of the actuator position from the angle sensor 15.

[0039] S4. Gear Shifting + Dual Closed-Loop Stepless Connection. Once the gear is engaged, the VCU employs cascaded dual closed-loop control consisting of an outer loop for vehicle speed and an inner loop for drive motor torque. The outer loop receives the driver's target speed and the actual vehicle speed, calculates the motor's target setpoint using PID control, and outputs the setpoint. The inner loop receives the setpoint command from the outer loop and, combined with feedback on the motor's actual speed and torque, achieves rapid motor adjustment. The working and transport gears are configured with independent PID parameter groups. The working gear range uses low-speed, high-torque PID parameters, suitable for plowing, deep tillage, and heavy-duty operations; the transport gear range uses medium-high-speed, high-efficiency PID parameters, suitable for transfer and light-duty operations. The overlapping speed ratio ranges are seamlessly connected through a smooth interpolation algorithm based on the motor's target speed—that is, in the overlapping area between the highest speed in the working gear and the lowest speed in the transport gear, the VCU performs linear or S-shaped interpolation to transition the drive motor's target speed, ensuring a smooth, stepless speed transition for the entire vehicle.

[0040] S5. Hybrid Torque Compensation Restores Power. This system is a series hybrid architecture. The engine drives a generator solely to produce electricity, and there is no mechanical transmission connection between the engine and the wheels. The drive motor is located on the input side of a two-speed synchronized gearbox, and power is transmitted to the wheels via the synchronized gearbox. During the shift window from S2 (torque reduction and disengagement) to S4 (gear engagement), synchronizer 6 disengages, temporarily disconnecting the mechanical transmission path between the drive motor and the wheels. The mechanical torque of the drive motor cannot be transmitted to the wheels, resulting in a drop in wheel-end traction. The following torque compensation strategy is implemented to suppress shift speed drop and jerking, and reduce the drop in traction.

[0041] Before shifting begins, the VCU records the target wheel-end traction force required by the entire machine based on the throttle opening and real-time load. After disengaging, the VCU controls the drive motor to exit torque output mode and switch to speed closed-loop control, pre-adjusting the drive motor speed to the synchronous speed corresponding to the target gear, thus completing speed alignment preparation in advance. During the shifting process, the generator power output and the power battery bus power are coordinated to maintain the DC bus power status. When the angle sensor 15 detects that the synchronizer 6 is engaged and the mechanical transmission path is re-established, the VCU immediately controls the drive motor to quickly output the preset target torque, rapidly restoring the wheel-end traction force. By pre-synchronizing the speed in advance, the total shifting time is shortened, reducing the depth and duration of traction force drop, achieving continuous traction force without significant drops during the shifting process. After the gear is completely stable, the system exits the shift compensation logic and returns to the dual closed-loop speed control corresponding to the gear.

[0042] S6. Power-off self-locking anti-gear disengagement. After the gear position is stable, the VCU sends a power-off command to the servo motor, relying on the self-locking characteristic of the thread pair of the ball screw pair to lock the gear position, preventing gear disengagement when coasting on slopes or bumpy in the field.

[0043] The shifting mechanism of this invention uses only one set of electric push rods in conjunction with segmented shifting slides and shift forks to achieve three-position shifting. It has an extremely simple structure, high reliability, and a mechanical self-locking function in the event of power failure. Based on the shifting mechanism, combined with segmented closed-loop speed regulation of the motor, it can achieve seamless stepless speed change of the tractor from zero to the maximum speed, combining the high efficiency of mechanical transmission with the stepless smoothness of electric drive. The control strategy of generator shifting voltage stabilization preprocessing + drive motor timing compensation proposed in this invention can effectively alleviate the speed drop and jerking problems during the shifting process of series hybrid tractors and reduce the drop in traction force.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A shifting mechanism for a series hybrid tractor, housed within a gearbox housing, comprising a power input shaft and a power output shaft, wherein the power input shaft is connected to a drive motor via a spline, a working gear drive gear and a moving gear drive gear are fixedly connected to the power input shaft, a working gear driven gear and a transport gear driven gear are rotatably connected to the power output shaft, and a synchronizer is mounted on the power output shaft between the two driven gears via a spline connection, characterized in that: The shifting mechanism also includes an electric push rod, a shift fork, and a section shift slide. The electric push rod is driven by a servo motor and communicates with the vehicle controller (VCU). The section shift slide is mounted on the gearbox housing. One end of the shift fork is fixed to the section shift slide by a shift fork locking nut, and the other end is connected to a synchronizer. The push block at the output end of the electric push rod is connected to the section shift slide outside the shift fork. The electric push rod has three mechanical limit strokes: neutral, working, and transport. During shifting, the electric push rod drives the shift fork and the section shift slide to move synchronously, driving the synchronizer to slide axially until it engages with the driven gear of the working gear or the transport gear, thus realizing gear switching.

2. The shifting mechanism for a series hybrid tractor according to claim 1, characterized in that, The electric actuator has a built-in angle sensor, which can identify the work position of neutral, working, and transport modes across the entire range.

3. The shifting mechanism for a series hybrid tractor according to claim 1, characterized in that, The section shift slide has three hemispherical limiting grooves along the axial direction, corresponding to the working gear, neutral gear and transport gear respectively. The positioning ball of the slide is pressed into the limiting groove by the positioning spring installed on the gearbox housing, so as to realize the independent rigid locking of the three gears.

4. The shifting mechanism for a series hybrid tractor according to claim 1, characterized in that, The electric actuator outputs linear motion through a ball screw thread pair and achieves self-locking through a ball screw self-locking nut.

5. The shifting mechanism for a series hybrid tractor according to claim 1, characterized in that, The power input shaft is a hollow shaft, and a PTO drive shaft passes through the inside of the power input shaft. The PTO drive shaft is directly connected to the engine, and the reverse gear of the hybrid tractor is achieved by reversing the drive motor.

6. A shift control method for a series hybrid tractor, characterized in that, The shift control method is implemented according to the shift mechanism of any one of claims 1-5, and includes the following steps: S0, stepless speed regulation zone prediction; VCU automatically determines the current optimal speed regulation zone based on the driver's throttle intention, real-time load, and motor speed: low speed heavy load automatically enters the working speed regulation zone, and high speed light load automatically enters the transport speed regulation zone. S1. Hybrid shift voltage regulation preprocessing: Before shifting, the VCU sends a command to the generator controller to increase the generator output power to stabilize the DC bus voltage and avoid bus voltage drop and power fluctuation during the shifting process. S2, Active torque reduction and synchronous disengagement: The VCU sends a torque unloading command to the drive motor controller to reduce the output torque of the drive motor; then it sends a command to the electric actuator to push it back to the neutral position with a light push force. S3, graded thrust engagement; the VCU controls the electric actuator to operate with three levels of variable thrust according to the target gear position: the first level of light thrust is used in the neutral shifting stage; the second level of medium thrust is used for flexible engagement in the synchronous engagement stage; and the third level of large thrust is used to ensure complete engagement in the gear position stage. The three levels of thrust are achieved by the VCU controlling the output torque of the servo motor in segments according to the real-time feedback of the actuator position from the angle sensor. S4, Gear shifting + dual closed-loop stepless connection; After the gear is in position, the VCU adopts cascade dual closed-loop speed regulation control of vehicle speed outer loop and drive motor torque inner loop. The working gear and the transport gear are configured with independent PID parameter groups respectively. The two speed ratio overlap ranges are seamlessly connected through the smooth interpolation algorithm of the motor target speed. S5, Hybrid torque compensation; Before shifting gears, the VCU pre-stores the target traction force at the wheel end. After disengaging the gear, the drive motor switches the speed closed-loop and pre-adjusts to the synchronous speed of the target gear. During the shifting process, the bus power is kept stable. After the gear is engaged, the drive motor quickly outputs the preset torque to restore the traction force. After the gear is fully stable, the system exits the shift compensation logic and returns to the dual closed-loop speed control corresponding to the gear. S6. Power-off self-locking anti-disengagement: After the gear position is stable, the VCU sends a power-off command to the servo motor, relying on the self-locking characteristics of the ball screw thread pair of the electric actuator to prevent disengagement.

7. The shift control method for a series hybrid tractor according to claim 6, characterized in that, In step S2, when a top tooth stall fault is detected, the push rod small reciprocating fine adjustment + motor micro speed disturbance fault tolerance logic is automatically executed, and the output drives the motor micro speed disturbance to finely adjust the gear phase until the synchronizer sleeve and the engagement tooth are smoothly aligned and meshed.

8. The shift control method for a series hybrid tractor according to claim 6, characterized in that, The dual closed-loop speed control is as follows: the outer loop receives the driver's target vehicle speed and the actual vehicle speed, and outputs the target setpoint value for the motor through PID calculation; the inner loop of the drive motor receives the setpoint command output by the outer loop, and combines the actual motor speed and torque feedback to realize the rapid adjustment of the motor.

9. The shift control method for a series hybrid tractor according to claim 6, characterized in that, When using dual closed-loop speed control, the low-speed, high-torque PID parameters are used in the working range, which is suitable for plowing, deep tillage, and heavy-load operations; the medium-speed, high-efficiency PID parameters are used in the transport range, which is suitable for transshipment and light-load operations.

10. The shift control method for a series hybrid tractor according to claim 6, characterized in that, In step S4, in the overlapping area between the highest speed in the working gear and the lowest speed in the transport gear, the VCU performs linear or S-shaped interpolation transition on the target speed of the drive motor, so that the whole vehicle has no speed step and no jerking.

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