Design of electric vehicle power management communication system based on CAN bus

Abstract: The power management scheme of electric vehicles involves many parameters such as the working conditions of the engine, motor, battery, vehicle speed, driving resistance, and driver's operation. Using CAN bus technology to connect the measurement and control devices of the above parameters is a key step in realizing the power management of electric vehicles. This paper mainly discusses the design and implementation technology of the communication system in the power management of electric vehicles based on CAN bus.

Keywords: electric vehicle; power management; CAN bus; communication technology

With the rise of oil prices and the improvement of environmental protection requirements, electric vehicles have become an important direction for the development of future vehicles. For battery-powered all-electric power systems or engine and battery hybrid power systems, the design of the power management system is an important factor related to vehicle performance. The design needs to consider the overall design of the vehicle and the external use environment. In order to save power, it is also necessary to design a certain control strategy to ensure the best use of power. Therefore, it is necessary to conduct an in-depth discussion on the power management system of all-electric vehicles.

1. The importance of energy management of electric vehicles
The power management of electric vehicles mainly aims to give full play to the combustion efficiency of fuel, make the engine work near the optimal operating point, and timely adjust the matching relationship between the vehicle operating conditions and external road conditions through the energy storage and output of the motor and battery. After more than ten years of development, the most practical and commercially viable model for electric vehicle powertrain design is hybrid vehicles. The hybrid powertrain assembly has evolved from the original discrete structure of the engine and motor to an integrated structure of the engine, motor and gearbox, that is, an integrated hybrid powertrain system. Therefore, only the power management of the hybrid powertrain is considered here. From a functional perspective, the power management of the hybrid powertrain needs to achieve the following two goals:

(1) Ensure the optimal operating condition of the engine and avoid inefficient operation of the engine. The engine can usually be adjusted to operate stably near the optimal operating point, and the output of the battery and motor can be adjusted to adapt to various external road conditions. For example, when the vehicle is in low-speed, gliding, or idling conditions, the battery pack drives the motor. When the vehicle is starting, accelerating, or climbing, the engine-motor group and the battery pack jointly provide power to the motor. In this way, the engine avoids idling and low-speed operation, thereby improving the efficiency of the engine, which not only reduces exhaust emissions but also saves power.

(2) Make full use of the vehicle's inertial energy. When the vehicle slows down, brakes or travels downhill, the inertia of the wheels drives the motor. At this time, the motor becomes a generator, which can reverse charge the battery and save fuel.

Statistics show that under more than 80% of road conditions, an ordinary car only uses 40% of its power potential, and it will drop to 25% in urban areas. Electric vehicles that use power optimization management, such as Toyota's Prius, have a power level that exceeds that of vehicles of the same level, saving 75% of fuel.

2. Communication requirements of power management system and CAN bus technology

The power management of electric vehicles needs to monitor the working conditions of the engine, motor, battery, vehicle speed, driving resistance data and driver's operation at any time, and can automatically control the energy-saving device or circuit operation after intelligent processing based on the above data. Therefore, it is necessary to first solve the connection method of the operating status sensor of components related to energy consumption and energy conversion.

At present, the data communication between the internal measurement and execution components of the car mainly adopts CAN bus technology. This bus technology was first introduced by BOSCH of Germany and is mainly used to solve the data exchange problem between many control and test instruments in modern cars. The electric vehicle power management system developed using the CAN bus not only has high communication speed, accuracy, and high reliability, but is also easily compatible with the vehicle control network, providing a basic platform for sharing sensor signals, calculation information and operating status of each control unit, and on-board or off-board fault diagnosis. Therefore, in this project, the CAN bus is used as the basic communication technology for power management.

3. Topology of energy management system based on CAN bus

The topology of the energy management network based on CAN bus formed by connecting the energy consumption and energy-saving systems of the chassis of electric vehicles is shown in Figure 1, which includes several lower key monitoring nodes such as brake energy conversion device, powertrain, battery management, motor controller, driving resistance test, and an upper master control node composed of an on-board computer system.

Figure 1 Topology of energy management network based on CAN bus

The brake energy conversion device works together with the driver's control monitoring system and the battery motor controller. When the driver steps on the brake pedal, the brake motor first approaches the rotating device to be braked, such as the transmission shaft, consumes the vehicle's inertia energy and converts it into electrical energy. At the same time, when the control monitoring system detects the action of the brake pedal, it adjusts the battery charging circuit to store the electrical energy transmitted by the brake motor.

The powertrain system is mainly used to achieve the optimized operation of the engine working condition. Under normal driving conditions, the energy of the engine is divided into two paths. One path is transmitted to the vehicle transmission and propulsion system to drive the vehicle to drive normally, and the other path drives the motor to work and supply power to the battery. At this time, the auxiliary power system composed of the motor and the battery is equivalent to an energy regulation device. Through the battery motor controller and the driving resistance test device, the adjustment and distribution of the two-way output energy of the engine are realized according to the changes in the external road conditions.

Through the CAN bus, the upper master control node composed of the on-board computer system connects the entire energy management and control network. Through a special software system, data collection, data analysis and control strategy output are carried out to achieve the optimal matching between the external driving resistance and the engine energy adjustment, realize the energy conversion and utilization inside the vehicle, and realize the energy saving, energy storage and energy replenishment regulation of the motor and battery system.

4. Structure and communication process of each monitoring node based on CAN bus

CAN bus node structure is generally divided into two categories: one is connected to the PC by using CAN adapter card to realize the communication between the host computer and CAN bus; the other is composed of single chip microcomputer, CAN controller and CAN driver, as a node to transmit data with CAN bus. In the energy management and control system designed in this paper, the upper master control node adopts the first type of CAN bus node structure, and each key monitoring/control system adopts the second type of CAN bus node structure. The structure of each node and the connection method of the system are shown in Figure 2. Two 120Ω resistors are configured at both ends of the bus, which is used to match the bus impedance, increase the stability and anti-interference ability of the bus transmission, and reduce the error rate in data transmission.


For each lower monitoring node, the 51 series single chip microcomputer can usually be used as the primary processing center device of the monitoring signal of the node, and SJA1000 is used as the CAN controller. PCA82C250 is a commonly used CAN transceiver and physical bus interface, which can mainly provide differential transmission capability for the bus and differential reception capability for the CAN controller. The circuit diagram of a lower-level monitoring node formed by the above three components is shown in FIG3 .