1. Structure of Vehicle Control Unit (VCU)
VCU is the dispatch and control center of a pure electric vehicle, responsible for communicating with other vehicle components and coordinating the overall vehicle operation. The VCU system structure is shown in the figure below.
It mainly includes a power supply circuit, digital input/output module, analog input module, and CAN communication module.
The power supply module takes power from the on-board 12V storage battery. The signals received by the digital input module mainly include key signal, gear signal, brake switch signal, etc.
The digital output signals are mainly used to control relays, whose functions vary slightly in different vehicle systems. Generally, they control relays such as water pump relay and PTC relay.
The analog input module collects signals such as accelerator pedal and brake pedal opening signals, as well as storage battery voltage signals.
The CAN module is responsible for communicating with other vehicle equipment, mainly including Motor Control Unit (MCU), Battery Management System (BMS), and charger.
2. Vehicle Communication Network Management
The vehicle system connects each sub-control system through the CAN communication network. The vehicle system communication network structure is shown in the figure below. The VCU coordinates and manages the entire communication network, acting as the communication server for each sub-device.
3. Vehicle Operating Mode Control
According to different vehicle working conditions and powertrain states, the following operating modes are divided:
1. Self-Test Mode
When the key switch signal is in the ON position, the self-test mode is activated, the main relay is closed, and the VCU performs self-test simultaneously. If the self-test fails, the system enters the fault handling mode; if the self-test passes, it waits for the start signal.
2. Start Mode
The driver powers on the VCU by turning the key and other operations, and then wakes up other nodes on the CAN network to start working. After all vehicle equipment is started normally, the system enters the READY state, indicating that normal driving operations can be performed.
When the key switch signal is in the START position, the self-test mode is valid, the gear is in P position, and there is no start-prohibited fault, the high-voltage power-on procedure is executed. Meanwhile, the VCU sends high-voltage power-on request commands to the motor system, DC-DC converter, and air conditioning control system. If the motor system, DC-DC converter, and air conditioning control system detect no high-voltage faults, they feed back a high-voltage power-on permission command to the VCU. The VCU realizes the high-voltage power-on process by controlling the high-voltage pre-charging and main relays. After the high-voltage power-on is completed, the EV-Ready light on the instrument panel lights up, and the start mode is completed.
3. Starting Mode
As shown in the figure below, when the vehicle starts from a standstill without pressing the accelerator pedal (creep starting), the target value of the expected motor torque is a calibratable vehicle torque Start-T. When the vehicle speed V < V1, the torque rises at a certain slope to overcome the static friction resistance of the vehicle;
When the vehicle speed V > V2, the motor power is controlled to limit the vehicle speed within a reasonable range, and the output motor torque is processed through a filtering link for smoothing, realizing smooth electric starting.
4. Driving Mode
The driving mode mainly controls the motor torque command in real time according to the accelerator pedal position and vehicle driving state, so as to control the vehicle operation in accordance with the driver's intention. The control methods in the driving control process are divided into constant torque control and constant power control, as shown in the figure below. The control output of the VCU is torque, and power is the constraint condition.
When the motor output power does not reach the expected power, the VCU adopts the constant torque control strategy. For the expected torque T*(k), the output torque T(k) = T*(k);
When the motor output power reaches the expected power, the VCU adopts the constant power control strategy. The expected output power is P*(k). If the motor speed n(k) at time k > nk, power control is adopted, P(k) = P*(k), and the output torque T(k) = P(k)×9.55/n(k).
The VCU collects control signals from the driver (gear signal, accelerator pedal signal, vehicle mode, etc.), and calculates and outputs the driving torque to the MCU through algorithms according to system constraints, so as to control the vehicle operation. According to the different needs of the driver, it can realize operation modes such as creeping, forward driving, reverse driving, cruise control, normal driving, sport driving, and economic driving.
5. Braking Mode
Compared with traditional fuel vehicles, electric vehicles can realize energy recovery during braking. When the electric vehicle is in the braking state, the VCU calculates the required braking torque through state data collection. At this time, the drive motor switches from the operating mode to the generator mode to charge the power battery pack.
6. Regeneration Mode
The regeneration mode controls the motor to charge the battery under specific working conditions. According to the brake pedal state, it is divided into coasting regeneration and braking regeneration. The figure below shows the schematic diagram of the braking strategy:
The power generation torques adopted for coasting regeneration and braking regeneration: constant torque power generation is adopted when the vehicle speed V is greater than V2, the braking regeneration torque is T2, and the coasting regeneration torque is T1. When the vehicle speed V is greater than V1 and less than V2, the motor braking regeneration or coasting regeneration power generation torque decreases proportionally. When the vehicle speed is less than V1, the motor power generation is cancelled. Meanwhile, the motor power generation torque depends on the current vehicle speed and the motor's power generation capacity. Then the torque is output to the motor after being limited by the power permitted by the battery.
7. Parking Mode
The parking mode is activated when the driver turns off the key normally without any faults during vehicle operation. In this mode, the VCU controls the motor and battery system to power off, and then controls each accessory device to shut down, completing the automatic power-off process.
8. Fault Mode
Vehicle faults are generally divided into 2 levels (Level 1 fault and Level 2 fault). Faults come from terminal equipment such as VCU, BMS, air conditioner, or input sensing equipment such as accelerator pedal sensor. Here, Level 1 fault is defined as a serious fault, and Level 2 fault is defined as a general fault. When a Level 2 fault occurs in the vehicle system, the vehicle enters the limp-home mode, which is mainly realized by limiting the system output power. When a Level 1 fault occurs in the vehicle system, the entire vehicle system enters an emergency stop state.
9. Charging Mode
During charging, the charging gun is plugged in, the charger starts working, and the VCU is triggered to power on. After detecting the charging connection signal, the VCU monitors the current vehicle state. When charging is allowed, the BMS is started. Then the BMS communicates with the charger to start the charging process. The VCU continuously monitors the status information of the BMS and charger. If charging is in progress, the instrument controller displays the charging light and charging status. When a fault occurs during the charging process, the VCU will cut off the BMS relay in time to interrupt the charging process and prevent dangerous accidents.
10. Power-Off Mode
When the key switch signal is in the OFF position, the power-off mode is activated. The VCU controls the BMS system to disconnect the high-voltage relay according to the high-voltage power-off permission signals from the motor, air conditioner and other high-voltage systems. Meanwhile, the VCU confirms whether to delay power-off according to the temperature of the motor system. When the temperature drops to a certain range, the motor cooling water pump and cooling fan are turned off, and the main power relay is closed, completing the power-off process.
