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The Design Of Automatic Transmission For Electric Vehicles

Electric vehicles are powered by renewable and clean electric energy, which overcomes the problems of environmental pollution and resource shortage of traditional internal combustion engine vehicles; electric vehicle traction motors have a wider working range than traditional internal combustion engines, and the motor has constant torque at low speed and constant power at high speed. The characteristics are more suitable for vehicle operation requirements. However, the fixed ratio reducer has only one gear, so that the electric vehicle motor is often in the low-efficiency area, which not only wastes precious battery energy and reduces the driving range, but also increases the requirements for the traction motor.

The electric vehicle traction motor must provide high instantaneous torque in the constant torque area and high operating speed in the constant power area in order to meet the vehicle performance requirements such as high speed, climbing and acceleration. In order to give full play to the advantages of electric vehicles and reduce the requirements for power batteries and traction motors of electric vehicles, the transmission system of electric vehicles should be multi-speed.

The complicated shifting operation of manual transmission and the need to cut off the power source during the shifting process affect the driving performance and comfort of electric vehicles. Automatic transmission is the development trend of vehicle transmission. Compared with manual transmission, automatic transmission has higher vehicle safety and comfort performance. The dual-clutch automatic transmission based on the parallel shaft manual transmission not only inherits many advantages such as high transmission efficiency, compact structure and low price of the manual transmission; The advantages of good shifting quality such as automatic transmissions. Therefore, electric vehicles with two-speed dual-clutch automatic transmission have better vehicle performance.

Structural Principle Of Electric Vehicle Automatic Transmission

System structure schematic diagram

Figure 1 shows the schematic diagram of the system structure of the two-speed dual-clutch automatic transmission. It is centered on the transmission electronic control unit and receives signals from sensors such as brake pedal, selector switch, accelerator pedal, etc., and can use CAN bus technology to receive signals from the whole system. Signals from the vehicle controller, such as vehicle speed, motor speed and other signals. The transmission electronic control unit collects the current road condition information, sends out signal commands through a certain shifting rule, and controls the clutch actuator to operate the clutch separation and combination.

The principle of transmission structure

According to the driving performance of the vehicle, it is determined that the ideal Ⅱ pure electric vehicle transmission design adopts a two-speed transmission to meet the power and economic requirements of the whole vehicle. Figure 2 is the transmission structure diagram of the two-speed dual-clutch automatic transmission. The output shaft of the clutch CL1 is a solid shaft, and a hollow shaft is sleeved outside the solid shaft, that is, the output shaft of the clutch CL2. The concentricity of the two output shafts makes the structure more compact. Through the motor and the deceleration and distance-increasing structure, the engagement and analysis of the two clutches realizes two-speed automatic transmission, and there is no need to add a shifting mechanism, which simplifies the structure of the system.

When the electric vehicle is stopped, the clutch CL1 and the clutch CL2 are both in the disengaged state, so no power is transmitted. When the electric vehicle starts, the automatic transmission electronic control unit controls the clutch CL1 motor to engage the clutch CL1. When the clutch CL1 is fully engaged, the electric vehicle enters the 1st gear. At this time, the clutch CL2 is still disengaged and does not transmit power. When the electric vehicle accelerates and reaches the shift speed value of 2nd gear, the clutch motor is controlled by the transmission electronic control unit so that the clutch CL1 starts to be disengaged, and the clutch CL2 starts to be engaged.

The two clutches are switched alternately until the clutch CL1 is completely disengaged and the clutch CL2 is fully engaged, and the electric vehicle upshifting process ends. When the electric vehicle enters the 2nd gear speed operation, the transmission electronic control unit collects relevant signals and judges whether the gear that the electric vehicle is about to run in is downshifted. The downshift process only needs to disengage the positive clutch CL2 and engage the disengaged clutch CL1 at the same time. Cooperate with the switching sequence of the two clutches, and shift according to a certain shifting pattern, and the entire shifting can be completed in an orderly manner.

The Principle Of Control System

The hardware principle of the control system

The automatic transmission electronic control unit TCU is the control core of the entire dual-clutch automatic transmission system. The design of the TCU is directly related to the quality and performance of the entire dual-clutch automatic transmission. This design selects Freescale’s 16-bit MC9S12C64 microcontroller. MC9S12C64 has high-speed digital signal processing capability, strong real-time performance, low power consumption, and high integration performance. Its working environment temperature can be between -40 ~ 125, which can overcome the impact of the harsh working environment of the Automotive field on the performance of the single-chip microcomputer.

MC9S12C64 adopts 16-bit microprocessor S12CPU, which has high computing and processing capabilities. MC9S12C64 has 2KB internal RAM, 64KB internal FLASH; an 8-channel 16-bit TIM (timer) module, which has the ability to efficiently process multiple time events, and meets the multi-channel speed signal acquisition requirements in the automatic transmission control system of electric vehicles; 8 channels 10-bit AD (analog-to-digital conversion) module, which meets the requirements of multi-channel analog signal conversion accuracy; 6-channel 8-bit PWM (pulse width modulation) module, which can meet the multi-channel motor drive signal output requirements of electric vehicles; CAN bus module for communication. Other available resources: The 80-pin package has 60 general-purpose I/O ports, two 8-bit bidirectional digital I/O ports, an internal watchdog, and more.

Taking MC9S12C64 as the core TCU system is mainly composed of main controller module, input module, motor drive module, display and CAN communication module. Signal input includes analog signal, switch signal and pulse signal input. Among them, the motor speed adopts a Hall pulse sensor, and the pulse signal output by the speed sensor is input to the pin of the single-chip microcomputer for speed measurement after photoelectric isolation and level conversion. The motor drive module uses PWM to control the speed and steering of the two clutch control motors. PWM has the advantages of high speed regulation accuracy, fast response speed, wide speed regulation range and low consumption.

The CAN communication module can transmit data between the TCU and the vehicle motor controller and anti-lock braking system, thus realizing the control of the motor speed and torque by the dual-clutch automatic transmission system, which not only simplifies the design, but also reduces the system manufacturing cost , while improving the integration and reliability of the TCU, improving the quality of shifting and the power and comfort of the vehicle. The display module adopts two digital tubes to display the gear position and fault code respectively. The fault alarm is realized by light-emitting diode and buzzer.

MC9S12C64 does not have internal EEPROM and clock, so it needs to add clock circuit and power-down memory. 

Software principle of conrol system software

The dual-clutch automatic transmission software system adopts the modular programming method, which is composed of main program, signal cnc machining, shifting decision-making, execution and other modules. The main program is a cyclic program, which continuously reads the vehicle status by collecting signals, calls the corresponding subroutines for shifting decision-making, shifting and fault diagnosis according to the driver’s operation in real time, and displays the gear position and fault code. Call the police.

The main program of the TCU system consists of the power-on initialization subroutine, the parking gear processing subroutine, the neutral gear processing subroutine, the forward gear processing subroutine, the reverse gear processing subroutine, and the online fault diagnosis and display program. The power-on initialization subroutine is mainly to initialize the software operating environment.

Subroutines such as parking gear machining subroutine, neutral gear processing subroutine, forward gear processing subroutine and reverse gear processing subroutine respectively form their own loops according to their respective gear processing characteristics and functions. The fault diagnosis and display function handles the abnormal or unhandled situation of the program, and displays the fault code. 

Advantages Of Automatic Transmission For Electric Vehicles

According to the structural characteristics of pure electric vehicle two-speed dual-clutch automatic transmission, it has the following advantages:

  • The structure is simple. The pure electric vehicle motor can steplessly adjust the speed within a certain range, so the transmission system can meet the dynamic performance and economy of the vehicle as long as it adopts two forward gears, and simplifies the transmission structure; Combination and separation control can meet the requirements of automatic transmission without shifting actuators.
  • Cost saving. The dual-clutch automatic transmission has a good inheritance of the original automatic transmission production line, and the investment in technical transformation is low; the complex mechanism such as the hydraulic system has been cancelled, and the manufacturing cost has been reduced; the battery and motor of pure electric vehicles are relatively expensive, and the two-speed dual-clutch automatic transmission can be used. Save battery power and reduce motor performance requirements, thereby saving vehicle manufacturing costs.
  • The shifting quality is high. The dual-clutch automatic transmission is simple in structure, stable in operation, does not cut off power during shifting, and has good shifting quality and transmission efficiency.
  • Easy maintenance and low cost. There is no hydraulic system, and complex structures such as shift actuators are eliminated, reducing maintenance difficulty and costs.


Under the social background of energy saving and emission reduction, electric vehicles will have good development prospects. Limited by battery technology and vehicle motor technology, electric vehicles need to use and will still need to use more than two-speed transmissions for a period of time in the future to meet the power and economy of the whole vehicle.

The two-speed dual-clutch automatic transmission for electric vehicles not only reduces the requirements of electric vehicles for batteries and motors, but also overcomes the problems of poor shifting quality of manual transmissions and the power source interruption of AMT shifting. Therefore, the two-speed dual-clutch automatic transmission has better performance. application prospects.

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