AMT gear shifting control method in combined braking constant-speed downhill process of electric truck

Abstract

The invention belongs to the technical field of new energy automobile transmission gear shifting control, and discloses an AMT gear shifting control method in an electric truck combined braking constant-speed downhill process, which comprises the following steps: judging whether a vehicle enters a combined constant-speed control state or not, and if the vehicle enters the combined constant-speed control state, selecting an AMT gear shifting rule based on a preset AMT gear shifting rule; the AMT optimal gear is obtained according to the current vehicle speed and the required regenerative braking torque, the limitation that only the motor efficiency is considered in a traditional method is broken through, the transmission system and the battery charging and discharging efficiency are synchronously covered, meanwhile, a braking torque cooperative change mechanism is creatively adopted, and the optimal gear is obtained. In the gear shifting process, braking force distribution of the motor and the hydraulic retarder is coordinated according to the preset AMT gear shifting rule, vehicle speed fluctuation caused by power interruption is eliminated, the energy loss and speed control instability risks caused by frequent gear shifting are overcome, driving comfort is guaranteed through gear shifting hysteresis control and the minimum vehicle speed threshold value, and safe and energy-saving closed-loop control is formed.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy vehicle transmission shift control, and specifically relates to an AMT shift control method for an electric truck during a combined braking and constant speed downhill process. Background Art

[0002] Driven by the global energy transition, the transportation industry is undergoing an electrification revolution. As the core transportation capacity of the logistics system, the transition to new energy for heavy-duty trucks is of strategic significance for achieving carbon reduction across the industry. In today's complex geographical environment, mountainous roads account for over 30% of total roads, and safety control issues on long downhill sections have become a key technical bottleneck restricting the promotion of new energy heavy-duty trucks. These sections pose severe challenges to the vehicle's braking system: traditional friction brakes are prone to failure due to thermal decay on long downhill sections, and while the regenerative braking system unique to new energy vehicles can recover energy, its effectiveness is constrained by the overall efficiency optimization of the powertrain system.

[0003] Current research focuses on the coordinated control of regenerative braking and mechanical braking, with the design of automatic mechanical transmission (AMT) shift schedules being a key component. Existing AMT regenerative braking shift strategies primarily prioritize motor efficiency. These strategies optimize the motor operating point to maximize motor operation within the high-efficiency range. A braking demand decomposition model is employed to allocate the ratio of regenerative braking to mechanical braking, and motor regenerative torque is controlled through gear adjustment. Shift point decisions rely on fixed threshold strategies that combine vehicle speed and slope parameters to determine shift timing, but these strategies fail to fully consider the dynamic changes in transmission efficiency within the transmission. In the area of ​​constant-speed downhill control, existing technologies often employ a proportional-integral-derivative (PID) closed-loop control algorithm to regulate brake pressure and regenerative braking power. However, the shift process is treated as an independent event and lacks coordination with the braking system.

[0004] However, existing technology systems suffer from key flaws: They focus solely on motor efficiency while ignoring variations in transmission efficiency, resulting in a loss of overall energy recovery efficiency. The switching between regenerative braking and mechanical braking during gear shifts creates a power vacuum, which can easily cause speed fluctuations on long downhill slopes. Constant-speed downhill control lacks a joint optimization model for gear decision-making and brake power distribution, leading to frequent gear shifting and the risk of brake overheating when responding to gradient changes. These technical shortcomings are particularly prominent in complex mountainous environments, hindering the large-scale application of new energy heavy-duty trucks in trunk logistics. Summary of the Invention

[0005] The present invention provides an AMT shift control method for an electric truck during combined braking and constant speed downhill, which solves the problem of frequent shifting or brake overheating risks when the system responds to slope changes.

[0006] To achieve the above object, the present invention provides the following technical solutions: The AMT shift control method for an electric truck during combined braking and constant speed downhill driving includes: Determine whether the vehicle has entered the joint constant speed control state; If the vehicle enters the combined constant speed control state, the optimal AMT gear is obtained based on the preset AMT shift schedule and the current vehicle speed and required regenerative braking torque. By comparing the current AMT gear, determine whether to shift gears. If the current AMT gear is the best AMT gear, keep the current AMT gear; otherwise, shift to the best AMT gear.

[0007] Preferably, the method for obtaining the preset AMT shift schedule is as follows: Get the transmission efficiency of each gear of AMT; Based on the transmission efficiency of each gear of AMT and with the goal of maximizing braking energy recovery per unit time, the optimal gear of AMT under different vehicle speeds and required regenerative braking torque is solved. The initial AMT shifting pattern is obtained according to the optimal gear of AMT, and the initial AMT shifting pattern is corrected to obtain the AMT shifting pattern.

[0008] Preferably, with the goal of maximizing braking energy recovery per unit time, the transmission ratio of the AMT optimal gear under different vehicle speeds and required regenerative braking torques is obtained, and then the AMT optimal gear is obtained based on the transmission ratio of the AMT optimal gear. The method for solving the transmission ratio of the AMT optimal gear under different vehicle speeds and required regenerative braking torques is:

[0009] Where, is the braking energy recovery per unit time, is the motor output torque, is the motor speed, is the motor efficiency, For battery charging efficiency, is the regenerative braking torque, is the transmission system efficiency, The transmission ratio of the AMT optimal gear is obtained according to the transmission ratio of the AMT optimal gear. is the main reducer transmission ratio, To maximize braking energy recovery per unit time.

[0010] Preferably, the steps of correcting the initial AMT shift schedule are as follows: Modify the initial AMT shift schedule based on the relationship between vehicle speed and the minimum speed of the combined constant speed control; Modify the initial AMT shift schedule based on the vehicle speed fluctuation threshold; Perform shift hysteresis control to complete the correction of the initial AMT shift schedule.

[0011] Preferably, the initial AMT shift schedule is modified according to the relationship between the vehicle speed and the minimum vehicle speed of the combined constant speed control as follows:

[0012] Where, is the current vehicle speed, It is the minimum vehicle speed for combined constant speed control.

[0013] Preferably, the initial AMT shift schedule is modified according to the vehicle speed fluctuation threshold as follows: The vehicle speed fluctuation threshold is 2km / h. When the vehicle speed fluctuates within 2km / h, the gear position will not be adjusted.

[0014] Preferably, the gear shift hysteresis control is specifically as follows:

[0015] Where, and They are respectively the regenerative braking torque threshold value for upshifting and the regenerative braking torque threshold value for downshifting; is the shift hysteresis threshold, is the regenerative braking torque.

[0016] Preferably, the steps of obtaining the transmission efficiency of each gear of the AMT are specifically as follows: Through the AMT bench test, the torque and speed data of the input / output shaft are obtained, the input power and output power are obtained, and then the transmission efficiency of each gear of the AMT is obtained.

[0017] Preferably, the steps of shifting to the optimal gear of the AMT are: The AMT controller takes over the control of the drive motor from the vehicle controller, controls the drive motor torque to drop to 0, controls the shift actuator to shift the transmission gear to the neutral position, controls the drive motor speed so that the input and output speeds of the shift engagement sleeve remain synchronized, controls the shift actuator to shift the transmission gear from neutral to the AMT optimal gear, controls the drive motor torque to increase to the target torque, and returns the motor control to the vehicle controller, completing the gear shift.

[0018] Preferably, the torque change rates for controlling the drive motor torque to drop to 0 and for controlling the drive motor torque to rise to the target torque are set to -3.5 kN·m / s and 4.5 kN·m / s, respectively.

[0019] Compared with the prior art, the present application has the following beneficial effects: the present application provides an AMT shift control method in the process of electric truck combined braking constant speed downhill, which significantly improves the brake energy recovery efficiency by dynamically matching the optimal gear, breaks through the limitation of traditional methods only considering motor efficiency, synchronously covers the transmission system and battery charging and discharging efficiency, and innovatively adopts a brake torque cooperative change mechanism to coordinate the brake force distribution of the motor and the hydraulic retarder in the shift process according to a preset AMT shift rule, and eliminates the speed fluctuation caused by power interruption. This double optimization makes the vehicle have energy saving and driving stability when the vehicle is in long downhill combined constant speed control, overcomes the energy loss and instability risk caused by frequent gear shifting, and guarantees driving comfort through shift hysteresis control and the lowest vehicle speed threshold, forming a safe and energy-saving closed-loop control.

[0020] The triple correction mechanism cooperatively improves the constant speed downhill control quality, the vehicle speed fluctuation threshold is locked when the vehicle speed is lower than the safety threshold to maintain the original gear, avoiding invalid shifting and energy loss in the low-efficiency working condition area, guaranteeing the stability of the basic control, the vehicle speed fluctuation tolerance control freezes the gear adjustment with ±2km / h as the fluctuation boundary to effectively filter out the false shifting caused by sensor noise and road disturbance, the shift hysteresis control establishes a buffer zone when the energy recovery benefits of the optimal and suboptimal gears are close by setting the upshift / downshift torque threshold and the hysteresis interval, reduces the energy loss caused by power interruption by reducing the shift frequency, and balances economy and comfort, and these correction strategies form a progressive optimization chain, the vehicle speed fluctuation threshold establishes a safety bottom line, the vehicle speed fluctuation tolerance improves the system anti-interference ability, and the hysteresis control realizes the fine balance between energy saving and smoothness, and achieves the double breakthroughs of vehicle speed fluctuation suppression and energy recovery gain in long downhill verification. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the AMT shift control method flowchart in the process of electric truck combined braking constant speed downhill of the present application; Figure 2 is the AMT transmission efficiency diagram of some six-gear embodiment of the present application; Figure 3 is the AMT shift rule optimization result diagram of the embodiment of the present application, and the right side diagram is a local enlarged diagram in the red box in the optimization result, and the numerical value therein represents a unit of time; Figure 4 is the AMT shift rule correction result diagram of the embodiment of the present application; Figure 5: This is a comparison diagram of the gear shifting control results during the combined constant speed descent process of an embodiment of the present invention, wherein a is a comparison of the vehicle speeds under normal gear shifting and the coordinated gear shifting control proposed by the present invention; b is a comparison of the braking torque under normal gear shifting and the coordinated gear shifting control proposed by the present invention; c is the motor efficiency and recovered energy before and after the gear shift; and d is the motor operating point before and after the gear shift. DETAILED DESCRIPTION

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0023] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0024] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0025] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0026] like Figure 1 As shown, the present invention provides an AMT shift control method for an electric truck during combined braking and constant speed downhill, comprising: The AMT shift control method for an electric truck during combined braking and constant speed downhill driving includes: S1: Determine whether the vehicle enters the joint constant speed control state; S2: If the combined constant speed control state is entered, the optimal AMT gear is obtained based on the preset AMT shifting rules and the current vehicle speed and required regenerative braking torque.

[0027] S3: Determine whether to execute gear shifting by comparing the current AMT gear. If the current AMT gear is the optimal AMT gear, maintain the current AMT gear; otherwise, shift to the optimal AMT gear.

[0028] The method for obtaining the preset AMT shift schedule is as follows: Get the transmission efficiency of each gear of AMT; Based on the transmission efficiency of each gear of AMT and with the goal of maximizing braking energy recovery per unit time, the optimal gear of AMT under different vehicle speeds and required regenerative braking torque is solved. The initial AMT shifting pattern is obtained according to the optimal gear of AMT, and the initial AMT shifting pattern is corrected to obtain the AMT shifting pattern.

[0029] By dynamically matching the optimal gear, the system significantly improves brake energy recovery efficiency, breaking through the limitations of traditional methods that only consider motor efficiency and simultaneously covering the efficiency of the drivetrain and battery charge and discharge. It also innovatively employs a coordinated braking torque variation mechanism, coordinating the braking force distribution between the motor and the hydraulic retarder during gear shifts using a preset AMT shift schedule, eliminating speed fluctuations caused by power interruptions. This dual optimization ensures both energy efficiency and driving stability on long descents combined with constant speed control. It overcomes the energy loss and speed control instability caused by frequent gear shifting, while ensuring driving comfort through shift hysteresis control and a minimum speed threshold, creating a safe and energy-saving closed-loop control system.

[0030] The detailed steps are: Determine whether the vehicle has entered the joint constant speed control state; If the vehicle enters the combined constant speed control state, the optimal AMT gear is obtained based on the current vehicle speed and the required regenerative braking torque based on the AMT shifting schedule. The current AMT gear is compared to determine whether to shift. If the current AMT gear is the optimal AMT gear, the current AMT gear is maintained; otherwise, the gear is shifted to the optimal AMT gear. If a gear shift is to be executed, first, the AMT controller takes over the control of the drive motor from the vehicle controller and controls the drive motor torque to drop to 0; secondly, the gear shift actuator is controlled to shift the transmission gear to the neutral position; thirdly, the drive motor speed is controlled so that the input and output speeds of the gear shift engagement sleeve are kept synchronized; then, the gear shift actuator is controlled to shift the transmission gear from neutral to the AMT optimal gear; finally, the drive motor torque is controlled to rise to the target torque, and the drive motor control is returned to the vehicle controller, and the gear shift is completed.

[0031] The torque and speed data of the input / output shafts are obtained through AMT bench testing, and the input and output powers are obtained, thereby obtaining the transmission efficiency of each gear of the AMT. Based on the transmission efficiency of each AMT gear and aiming to maximize brake energy recovery per unit time, the optimal AMT gear under different vehicle speeds and required regenerative braking torques is determined. The optimal AMT gear is then modified based on the principle of reducing shift frequency, ultimately resulting in the AMT shift schedule. Unlike existing AMT shift control methods that only consider motor efficiency when designing AMT shift schedules, the steps for obtaining AMT shift schedules are as follows: Firstly, the influence of transmission efficiency differences under different AMT gears on energy recovery effect is considered. With the goal of maximizing braking energy recovery, the vehicle speed and wheel-end required regenerative braking torque are used as inputs, and an exhaustive search method is used to solve the optimal AMT gear.

[0032] Then, considering the impact of frequent gear shifting on the combined constant speed control effect and energy loss, the optimized AMT shift schedule is corrected.

[0033] Different from the existing AMT shift control method that aims to shorten the shift time, the maximum allowable impact is used to constrain the torque change rate of the drive motor torque unloading and recovery stages to ensure the shift quality. The goal is to coordinate the control of the braking torque, aiming to minimize the impact of power interruption during shifting on the combined constant speed control effect. Combined with the dynamic characteristics of the hydraulic retarder's charging and discharging fluid, the torque change rate of the drive motor torque unloading and recovery stages is limited.

[0034] Another embodiment of the present invention provides an AMT shift control method for an electric truck during a combined braking and constant speed downhill process. According to the bench test results of a six-speed AMT transmission efficiency, as shown in FIG. Figure 2 As shown in the figure, there are significant differences in transmission efficiency under different gears, torques and speeds. Figure 2 In the figure, a through f correspond to gears 1 through 6, respectively. During regenerative braking, the more efficient the transmission system, the less energy is lost during energy transfer due to power losses in bearings and gear meshing. Therefore, the overall efficiency of the regenerative braking system should be considered when developing an AMT shift schedule.

[0035] First, the vehicle speed and wheel-end regenerative braking torque are used as inputs, and an exhaustive search method is used to solve the optimal gear position of the AMT under different conditions, with the goal of maximizing the braking energy recovery per unit time, as shown in the following formula: (1) Where, is the braking energy recovery per unit time, is the motor output torque (absolute value), is the motor speed, is the motor efficiency, For battery charging efficiency, is the regenerative braking torque, is the transmission system efficiency, The transmission ratio for the best gear of AMT, is the main reducer transmission ratio, To maximize braking energy recovery per unit time.

[0036] AMT gear optimization results, such as Figure 3As shown in the figure on the right, a zoomed-in view of the red box in the optimization results is shown. The numerical value represents the additional braking energy recovered per unit time in the optimal gear compared to the suboptimal gear. As can be seen from the figure, since the AMT's sixth gear has higher transmission efficiency than the other gears, it accounts for the largest proportion of the optimal gears. When the required regenerative braking torque is high, the motor output torque is lower and the speed is higher when the AMT is in a low gear, resulting in higher motor efficiency. At this time, the overall efficiency of the regenerative braking system is higher, resulting in better energy recovery.

[0037] However, due to the power interruption characteristic of AMT shifts, brake energy cannot be effectively recovered during the shift process. Therefore, excessive shifting during constant-speed downhill braking will cause brake energy loss and affect the vehicle's combined constant speed control. Therefore, based on the actual vehicle operating characteristics, the optimized AMT shift schedule was modified according to the following principles.

[0038] (1) When the vehicle speed is low, the motor may not be able to achieve brake energy recovery, and the effect of gear shifting on improving energy recovery efficiency is quite limited. Therefore, if the current vehicle speed is lower than the minimum speed of the combined constant speed control, the AMT maintains the original gear position unchanged, as shown in the following formula: (2) Where, is the current vehicle speed, It is the minimum vehicle speed for combined constant speed control.

[0039] (2) The speed fluctuation threshold during constant speed downhill control is 2 km / h. Figure 3 The vehicle speed interval on the middle horizontal axis is 2 km / h. To avoid unnecessary gear shifting, the number of gear changes within the speed fluctuation threshold (i.e., the data in the same column in the figure) should be reduced as much as possible.

[0040] (3) By Figure 3 As can be seen in the figure on the right, at the boundary of the gear optimization results, the difference in braking energy recovered per unit time between the optimal and suboptimal gears is small. At this point, the shifting energy gain is minimal and may even lead to frequent gear changes, affecting braking comfort and energy recovery efficiency. To this end, a shift hysteresis link is added, as shown in the following formula: (3) Where, and They are respectively the regenerative braking torque threshold value for upshifting and the regenerative braking torque threshold value for downshifting; is the shift hysteresis threshold.

[0041] AMT shift schedule correction results, such as Figure 4As shown in the figure, during combined braking and constant speed descent, the AMT shifts gears based on regenerative braking torque and vehicle speed, maintaining the original gear position when in the upshift and downshift hysteresis area. This improves the vehicle's brake energy recovery efficiency, reduces unnecessary shifting, and ensures driving comfort.

[0042] In the previous AMT shift control strategy, in order to ensure the shift quality, the maximum allowable impact The torque change rate during the motor torque unloading and recovery phases is constrained as shown in Equation (4). At the same time, in order to shorten the shift time, the motor is usually controlled to complete torque unloading and recovery at the maximum torque change rate (about 7 kN·m / s).

[0043] (4) in, is the motor output torque, The transmission ratio for the best gear of AMT, is the main reducer transmission ratio, is the motor change rate, For time, is the vehicle rotation mass conversion factor, For car quality, is the transmission efficiency of the transmission system, The maximum gear shift shock.

[0044] When the regenerative braking torque changes too rapidly during a gear shift during constant speed downhill control, the hydraulic retarder's output torque cannot respond in a timely manner, inevitably leading to significant vehicle speed fluctuations. To ensure effective combined constant speed control during the gear shift, the motor torque unloading and recovery rates are appropriately slowed so that the retarder's output torque can compensate for the loss of regenerative braking torque during the shift. Based on the dynamic characteristics of the hydraulic retarder's charging and discharging, the torque change rates during the motor torque unloading and recovery phases are set to -3.5 kN·m / s and 4.5 kN·m / s, respectively.

[0045] Finally, in order to verify the superiority of the invention, the proposed shift control method is compared with the non-shift and normal shift (based on the maximum impact Limiting the motor torque unloading and restoring the torque change rate) control method are compared.

[0046] A long downhill road with a variable slope (sinusoidal change, ranging from 4% to 6%) was selected as the verification condition. The target speed was 40 km / h. When the actual speed stabilized, the AMT shift control was executed in about 2 seconds, shifting from 6th gear to the AMT's optimal gear - 4th gear. The comparison results are as follows: Figure 5 shown.

[0047] Figure a compares vehicle speeds under no gear shifting, normal gear shifting, and the coordinated gear shifting control proposed by the present invention. As can be seen, under normal gear shifting, the motor torque resets and recovers more quickly, and the retarder output torque fails to respond in a timely manner, resulting in uncoordinated torque control and a significant fluctuation in vehicle speed between 2 and 3.2 seconds.

[0048] Figure b compares the braking torque during a normal shift and the coordinated shift control proposed by the present invention. As can be seen, during the motor torque unloading and recovery phases, the regenerative braking and retarder braking torque change rates are similar. While shift time increases, the issue of torque control uncoordination is effectively avoided, resulting in no significant change in vehicle speed during the shift.

[0049] Figure c shows the motor's working efficiency and brake energy recovery before and after the gear shift. It can be seen that the motor is in a more efficient area after the gear shift, and the motor efficiency is significantly improved. The power interruption during the gear shift causes the early recovery energy to lag behind. As the braking duration increases, the advantage of brake energy recovery gradually emerges.

[0050] Figure d shows the motor operating point before and after the gear shift. As can be seen from the figure, the motor torque and speed change after the gear shift, and the motor operating point shifts from a lower efficiency area to a higher efficiency area.

[0051] Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments and application fields. The above-mentioned specific embodiments are merely illustrative and instructive, and are not restrictive. A person skilled in the art, guided by the description, may devise various forms without departing from the scope of protection of the claims of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. AMT shift control method for electric trucks during combined braking and constant speed downhill, characterized in that: include: Determine whether the vehicle has entered the joint constant speed control state; If the vehicle enters the combined constant speed control state, the optimal AMT gear is obtained based on the preset AMT shift schedule and the current vehicle speed and required regenerative braking torque. By comparing the current AMT gear, determine whether to shift gears. If the current AMT gear is the best AMT gear, maintain the current AMT gear; otherwise, shift to the best AMT gear.

2. The AMT shift control method for an electric truck during combined braking and constant speed downhill driving according to claim 1 is characterized in that: The method for obtaining the preset AMT shift schedule is as follows: Get the transmission efficiency of each gear of AMT; Based on the transmission efficiency of each gear of AMT and with the goal of maximizing braking energy recovery per unit time, the optimal gear of AMT under different vehicle speeds and required regenerative braking torque is solved. The initial AMT shifting pattern is obtained according to the optimal gear of AMT, and the initial AMT shifting pattern is corrected to obtain the AMT shifting pattern.

3. The AMT shift control method for an electric truck during combined braking and constant speed downhill driving according to claim 2 is characterized in that: The method for determining the optimal gear position for the AMT under different vehicle speeds and required regenerative braking torques is as follows: with the goal of maximizing braking energy recovery per unit time, the transmission ratio of the optimal gear position for the AMT under different vehicle speeds and required regenerative braking torques is obtained, and then the optimal gear position for the AMT is determined based on the transmission ratio of the optimal gear position for the AMT. The method for solving the transmission ratio of the AMT optimal gear under different vehicle speeds and required regenerative braking torque is: Where, is the braking energy recovery per unit time, is the motor output torque, is the motor speed, is the motor efficiency, For battery charging efficiency, is the regenerative braking torque, is the transmission system efficiency, The transmission ratio for the best gear of AMT, is the main reducer transmission ratio, To maximize braking energy recovery per unit time.

4. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 2 is characterized in that: The steps for correcting the initial AMT shift schedule are as follows: Modify the initial AMT shift schedule based on the relationship between vehicle speed and the minimum speed of the combined constant speed control; Modify the initial AMT shift schedule based on the vehicle speed fluctuation threshold; Perform shift hysteresis control to complete the correction of the initial AMT shift schedule.

5. The AMT shift control method for an electric truck during combined braking and constant speed downhill driving according to claim 4 is characterized in that: The initial AMT shift schedule is modified based on the relationship between the vehicle speed and the minimum speed of the combined constant speed control, specifically: Where, is the current vehicle speed, It is the minimum vehicle speed for combined constant speed control.

6. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 4 is characterized in that: The initial AMT shift schedule is modified according to the vehicle speed fluctuation threshold as follows: The vehicle speed fluctuation threshold is 2km / h. When the vehicle speed fluctuates within 2km / h, the gear position will not be adjusted.

7. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 4 is characterized in that: The gear shift hysteresis control is as follows: Where, and They are respectively the regenerative braking torque threshold value for upshifting and the regenerative braking torque threshold value for downshifting; is the shift hysteresis threshold, is the regenerative braking torque.

8. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 2, characterized in that: The steps to obtain the transmission efficiency of each gear of AMT are as follows: Through the AMT bench test, the torque and speed data of the input / output shaft are obtained, the input power and output power are obtained, and then the transmission efficiency of each gear of the AMT is obtained.

9. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 1, characterized in that: The steps to shift to the optimal gear for AMT are: The AMT controller takes over the control of the drive motor from the vehicle controller, controls the drive motor torque to drop to 0, controls the shift actuator to shift the transmission gear to the neutral position, controls the drive motor speed so that the input and output speeds of the shift engagement sleeve remain synchronized, controls the shift actuator to shift the transmission gear from neutral to the AMT optimal gear, controls the drive motor torque to increase to the target torque, and returns the drive motor control to the vehicle controller, completing the gear shift.

10. The AMT shift control method for an electric truck during combined braking and constant speed downhill descent according to claim 9, characterized in that: The torque change rates for controlling the drive motor torque to drop to 0 and for controlling the drive motor torque to rise to the target torque are set to -3.5 kN·m / s and 4.5 kN·m / s, respectively.

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