Abstract: A dual rear-wheel independent drive motion control system was implemented for low-speed ideal vehicle motion of electric vehicles (EVs). The vehicle is driven by two independently controlled hub-type brushless DC motors (BLDC) and DC servo motor power steering (EPS). The relevant theoretical analysis and experimental verification of straight-line driving and turning driving were carried out.
Keywords: Dual-wheel independent drive; Electric vehicle; Electric power steering; DC brushless electric drive
Research on electric vehicle movement controlling TANGWenyang, YOU Yiming, TAO Yin
0 Introduction
This paper designs and implements a motion control system for a model electric vehicle that combines electric power steering with independent rear-wheel drive, based on existing electric vehicle power control methods. This system integrates electric power steering with dual rear-wheel hub motor drive, eliminating traditional components such as clutches, transmissions, final drive, and differentials. This significantly simplifies the overall vehicle structure, greatly improves the electrification and controllability of the electric vehicle, and fully leverages the advantages of the highly integrated motor system. The paper details the design and control methods of each key subsystem and demonstrates the effectiveness of the design through bench tests.
1. Overall Composition of Model Electric Vehicle System
Designed for low-speed driving under ideal conditions in electric vehicles (EVs), this system implements a dual rear-wheel independent drive motion model. The system structure is shown in Figure 1. The front wheels of the model vehicle are controlled using an electric power steering (EPS) system, with power provided by two rear-wheel motors. The EPS drive uses ordinary DC servo motors, which are simple to control; the two rear-wheel motors are two hub-type brushless DC (BLDC) motors, which can improve efficiency while ensuring long-term operational reliability. Each motor in the system independently forms a speed closed-loop and a current closed-loop system with the electronic control unit (ECU). This design, while maintaining the driving feel of traditional cars, eliminates components such as clutches, transmissions, final drive, and differentials, greatly simplifying the overall vehicle structure, improving transmission efficiency, and enabling power steering and electronic differential control of the electric wheels through control technology.
2 Design of Motion Control System for Dual Rear-Wheel Drive Electric Vehicle
The motion control of prototype electric vehicles mainly needs to solve the following two problems: first, the power steering system control problem; second, the coordinated control problem of the two independent drive wheels.
2.1 Power Steering Control
The electric power steering process is as follows: First, the torque sensor measures the steering torque applied by the driver to the steering wheel, and the vehicle speed sensor measures the current vehicle speed. These two signals are then transmitted to the ECU. The ECU calculates the ideal target assist torque based on the built-in control strategy and converts it into a current command for the motor. Then, the assist torque generated by the motor is amplified by the reduction mechanism and acts on the mechanical steering system. Together with the driver's steering torque, it overcomes the steering resistance torque and achieves vehicle steering.
The assist motor control strategy adopts closed-loop current control of the assist motor, and its control function structure block diagram is shown in Figure 2.
This control structure simplifies the process of adjusting the actual assist characteristics, makes the adjustment of control parameters convenient and intuitive, and ensures economy while meeting control requirements.
2.2 Two-wheel drive control
It adopts a dual rear wheel independent drive scheme, where each drive wheel can provide driving force independently, and the power can be distributed independently as needed. Its differential function can be completed by software, realizing electronic differential.
To determine whether the driver's intention is to drive straight or turn, the steering wheel angle θ is a crucial parameter. The strategy introduces a flag value called the steering wheel free travel angle ε. When |θ| > ε, the vehicle electronic control unit (ECU) interprets the driving intention as turning; otherwise, it's straight driving. Regardless of whether it's a straight-driving control strategy or a steering control strategy, the key is to control the two motors by adjusting the target speeds ni1 and ni2, thereby controlling the vehicle's trajectory. The block diagram of the dual-motor coordinated control is shown in Figure 3.
1) Straight-line driving control strategy: During straight-line driving, the speeds no1 and no2 of the two motors are difficult to achieve perfect synchronization, and there will always be a certain speed difference Δn (defined as Δn = no1 - no2). The ECU needs to monitor Δn. When Δn exceeds the system's allowable real-time speed difference np, the target speeds ni1 and ni2 need to be adjusted based on Δn and np, with the adjustment amount being nin. To ensure stable straight-line driving, the ECU also needs to monitor the cumulative travel difference ΔS of the two motors. When ΔS exceeds the system's allowable real-time speed Sp, the target speeds ni1 and ni2 also need to be adjusted based on ΔS and Sp, with the adjustment amount being nis. nis is calculated based on the cumulative travel difference, nis = C3ΔS, where C is a proportional constant determined experimentally and cannot be too large, otherwise it may cause instability. The calculation result is used to adjust the input speed of the two motors to reduce the cumulative travel difference, thus achieving closed-loop control. Through the dual synchronization of cumulative travel and speed, the reliability of stable straight-line driving of the vehicle is enhanced.
2) Steering Control Strategy: During steering control, the ECU calculates the target speed difference m between the two motors based on the absolute value of the steering wheel angle θ. Depending on the sign of θ, it determines which of the two motors in the drive system is the outer motor and which is the inner motor. The target speed of the outer motor remains unchanged at the current speed, while the target speed of the inner motor should be reduced by m from the current target speed, thus achieving steering. Figure 4 shows the control flowchart for straight driving and steering.
3. Experimental Results
Based on the above control strategy, a control program for the vehicle electronic control unit (ECU) was written and put into operation for testing. Some experimental results are shown in Figure 5, and can be roughly divided into the following stages:
1) Before point a, the vehicle travels in a straight line.
2) Between points a and b, the driver quickly turns the steering wheel to the left to a large angle Θ, then keeps the steering wheel position unchanged, and the vehicle begins to turn left. The speed n1 of motor 1 remains unchanged, while the speed n2 of motor 2 is adjusted downwards until the target speed difference is reached.
3) Between points b and c, the steering wheel position remains unchanged from the previous stage, motor 1 and motor 2 maintain a stable speed difference, and the vehicle steers.
4) Between points c and e, the steering wheel returns to the middle position, and the driver intends to drive straight. At this time, the speed of motor 1 n1 is adjusted downward, and the speed of motor 2 n2 is adjusted upward. The two meet at point d; after the adjustment of segment de, they reach a basic consistency at point E.
5) The period between points e and f is an acceleration process that brings the vehicle speed up to the speed value before turning.
6) After point f, the vehicle continues to travel in a straight line.
Experimental results show that, through a coordinated control strategy based on angle and speed, the vehicle electronic control unit (ECU) can effectively control the motors on both sides, and promptly and accurately meet the driver's requirements for straight-line driving and steering.
4. Conclusion
A motion control model for electric vehicles combining electric power steering and dual rear-wheel drive technology was designed and implemented. A coordinated control strategy for the two hub motors, based on angle and speed control, was proposed, providing a solution to the stability problem of dual rear-wheel drive electric vehicles. Bench test results show that the control strategy can effectively meet the requirements of straight-line driving and steering control, proving the effectiveness of the design.
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