Abstract: Models for motor control and gear shifting control were established using the ADVISOR software environment. Aiming to improve the power performance and efficiency of electric drive systems in new energy vehicles, and combining the precise speed and torque regulation control characteristics of drive motors, a perspective based on the output torque powertrain transmission system was proposed from the integrated control link of the motor and gearbox. Simulations and experiments were conducted to verify this perspective.
1 Introduction
The control of electric vehicle motors is generally sufficient for vehicle operation on flat roads. However, certain special operating conditions are difficult to handle. For example, when the vehicle is going downhill, the motor load is very small, close to zero, and applying an acceleration signal may pose a safety hazard. Therefore, the speed must be continuously adjustable over a wide operating range. Instantaneous starting requires a large torque, which can easily demagnetize the permanent magnet motor. This can be addressed by increasing the speed ratio through a transmission mechanism to improve output torque. This paper considers the gearbox as a comprehensive coordinating component for output torque and motor speed. The entire powertrain system includes: a motor control system, a gearbox control system, a vehicle control system, and several functional submodules.
2 System Design Scheme and Model Establishment
The powertrain transmission system is based on a permanent magnet synchronous motor vector speed regulation system with the addition of a mechanical automatic transmission (AMT). A complete set of electronic control units optimizes and enhances vehicle power performance, as shown in Figure 1. The electronic control unit (TCU) serves as the core of the entire transmission control. In pure electric vehicles, the clutch control is eliminated in the gear selection and shifting mechanism. The control unit needs to collect acceleration and brake pedal signals and communicate with the shifting unit and motor control module. It also integrates the functions of the vehicle control unit (EVCU).
The electric vehicle model was simulated using ADVISOR software, as shown in Figure 2. A novel powertrain simulation model was constructed by incorporating a transmission model with a direct-drive motor, and simulation studies were conducted.
After adjusting the powertrain model in the electric vehicle simulation model, the terminal voltage of the gear selector motor is used as the input and the rotational speed of the motor is used as the output. The gear selector transfer function is derived and established: Figure 3 shows the simulation model of a two-speed gearbox.
During gear shifting, the current loop of the drive motor operates, and gear shifting control is achieved by adjusting the speed and torque parameters. Traditional torque control theories mainly include the following two types: (1) directly reducing the torque to zero; (2) reducing the torque to zero smoothly with a curve transition. The torque reduction control strategy adopted in this paper calculates the required torque before shifting and then combines the torque reduction curve to reach the predetermined value, as shown in Figure 4.
The required torque Te depends on parameters such as accelerator pedal Acc, brake pedal signal Bra, gear signal Ks, vehicle speed Vcar, drive motor speed Vmotor, battery pack voltage level U, current magnitude I, and battery load Qs.
The torque function can be broken down into the desired driving speed function Vexp and the required torque function Tened:
The drive motor control employs an embedded permanent magnet motor current and speed closed-loop vector control. The essence of vector control is to synchronize the rotating magnetic field with the stator magnetic field, with their phase angle relationship satisfying: the stator magnetic field lags the rotating magnetic field by π/2. Signal feedback is obtained from the drive motor's sensors, the PWM signal duty cycle is calculated, and the control algorithm is ultimately implemented. Based on the sensor-based speed regulation method of the permanent magnet motor, a variable frequency speed regulation method is proposed to improve the drive motor's output torque. Assuming the control cycle time is 2T, the corresponding waveform function f(x) is analyzed using Fourier expansion to study the harmonics of the current. To simplify the calculation, f(x) is made an even function.
In the above formula
f(x) is an even function
In the above formula, dpwm is the DC component and is linearly related to the duty cycle, which is also the main voltage component that generates motor torque. The remaining harmonics are sinusoidally distributed, fsw is the PWM frequency, and the harmonic frequencies satisfy integer multiples of fsw, varying with the fundamental frequency. Motors are inductive loads, and the inductor current characteristics are: DC passes while AC blocks, low frequencies pass while high frequencies block. Higher harmonic frequencies are higher, and the inductance effect is more pronounced. Appropriately reducing the PWM frequency during motor control lowers the harmonic frequency, increasing the system's harmonic components and thus increasing the average output torque of the motor. The control strategy utilizing harmonic torque is particularly suitable for increasing output torque at low motor speeds, improving the power performance of electric vehicle powertrain systems.
3. Analysis of System Simulation Results
As shown in Figure 5, the two-speed gearbox has a gear ratio of 3:1. In the shift motor model, some parameters are set as follows: armature inductance L = 0.8mH; armature resistance R = 0.5Ω; torque constant Kt = 0.05N·m/A; moment of inertia J = 0.2g·m²; back EMF constant Ke = 0.05v·s/rad. The motor's rated speed is set to 700r/min, and the peak output torque is 25N·m. The gear selection signal, shift signal, output speed, and motor torque are tracked. Based on the transfer function, we know that:
Figure 6 illustrates the operation of the selection motor and the shifting motor. The dashed line at 2.5 on the vertical axis represents the center point of the lead screw movement, the solid line represents the target position of the DC motor, and the dashed line represents the actual position of the selection/shifting motor. Initially, the shifting mechanism is in neutral. At 0.25 seconds, it shifts into first gear. After shifting, the drive motor speed and torque increase. When the shift point is reached, it shifts from first gear to second gear. During the shifting process, the drive motor torque decreases to the required torque. After a successful shift, the torque and speed increase again, and the powertrain output speed reaches the required value. At this point, the motor operates stably.
The simulation results of the motor vector control system were analyzed. From the obtained speed and torque waveforms, as shown in Figure 7, it can be roughly seen that the torque ripple of the motor control system is small, the response speed is fast, it is very sensitive and the fluctuation is very small, and the whole system can operate stably.
4-unit experiment and data analysis
The main structure of the experimental bench is shown in Figure 8. The entire experimental bench has low energy consumption and can perform loading actions according to a set program, recording the input speed, torque, output speed, torque, efficiency curves, etc. of the transmission. ① High-power cooling blower; ② High-efficiency embedded permanent magnet synchronous motor; ③ Domestic Fast Gearbox with integrated shifting mechanism; ④ Drive shaft; ⑤ Torque and speed sensor integrated box. To verify the motor speed regulation and torque-based shifting control strategy of the transmission system, the main tasks of the experimental bench are: shifting test, communication test, efficiency test, monitoring the powertrain system from start-up to stable speed operation, and the transmission controller and motor controller work collaboratively through CAN communication, generating corresponding data tables.
The main parameters of the test system are as follows: transmission assembly input torque range 0-200Nm; torque measurement accuracy ±0.05%FS; output signal frequency 60±30KHz; transmission input speed range 0-6500rpm; speed sensor accuracy ±1rpm; gear ratio reference transmission nameplate (1st gear 3.816, 2nd gear 2.15); drive motor (rated power 16KW, maximum torque 180Nm, rated speed 1800rpm, maximum speed 6000rpm). The motor's rated speed is 1800rpm, and the transmission gear ratios are 3.816 for 1st gear and 2.15 for 2nd gear. Output speed calculations show that in 1st gear, the motor reaches its rated speed at approximately 470rpm, and in 2nd gear, the motor reaches its rated speed at approximately 837rpm. The target speed is set to 1000rpm. With the powertrain accelerator pedal opening constant, efficiency curves are plotted for different vehicle speeds before reaching the target speed, specifically for 1st gear, 2nd gear, and when shift control is added.
As shown in Figure 9, the continuous automatic shifting curves indicate that the system can work normally. The efficiency curve of a single gear drops significantly after exceeding the reasonable speed operating range. The system efficiency using the control strategy in this paper is higher than that of working in a single gear, and the high-efficiency operating range is significantly larger.
5 Conclusion
This study completed the simulation and bench test of the powertrain transmission system of a torque-based pure electric vehicle. Under a vector closed-loop control algorithm, the permanent magnet synchronous motor's speed, torque, and three-phase current were tracked. The vehicle model was then adjusted and analyzed using ADVISOR software. Based on the bench test data, the powertrain efficiency curve was derived, verifying the significant role of the powertrain system control scheme in kinetic energy transfer, energy management, and increasing driving range.
