Abstract: Hybrid electric vehicles (HEVs) are automobiles driven by a combination of an internal combustion engine and an electric motor. Their main characteristics are energy saving and environmental protection. Using the internationally advanced CAN (Controller Area Network) bus as the communication medium, an intelligent multi-energy management unit as the control core, and five relatively independent intelligent nodes (engine control unit, motor drive unit, battery management unit, system data acquisition unit, and display unit) as auxiliary nodes, a network control system is constructed. Coupled with excellent control strategies, HEVs exhibit better performance than traditional automobiles. Keywords: Hybrid electric vehicle; CAN bus; Electronic control system Hybrid electric vehicles (HEVs) are automobiles driven by a combination of an internal combustion engine and an electric motor. Their main characteristics are energy saving and environmental protection. During start-up, the electric motor drives the vehicle, eliminating the black smoke emitted by the internal combustion engine due to incomplete combustion. During deceleration or braking, a generator converts kinetic energy into electrical energy, which is stored in the battery, achieving energy recovery and energy saving. Because this type of vehicle uses both an internal combustion engine and an electric motor, traditional electronic control systems designed only for internal combustion engines cannot achieve optimal coordination between the two power sources. Therefore, developing a completely new electronic control system for hybrid vehicles is essential. This article, using the successful implementation of a parallel hybrid electric vehicle as a case study, introduces the structure, function, and effects of the hybrid vehicle's electronic control system from a system perspective. A Brief Introduction to the Parallel Hybrid Drive Structure The drive system structure of a parallel hybrid electric vehicle is shown in Figure 1. The engine is connected to the drive axle via a mechanical transmission, and the electric motor is also connected to the drive axle via a power combination device. The vehicle can be driven by both the engine and the electric motor, or by each motor individually. The structure of a parallel hybrid electric vehicle is more like that of a regular internal combustion engine vehicle with an added electric motor drive system. The electric motor plays a "peak shaving" role: when the power required for the vehicle's operation exceeds the engine's power, the electric motor draws electrical energy from the battery to generate electromagnetic torque and provides additional drive power to the drive axle. Some parallel hybrid electric vehicles also have a generator, but its main function is to charge the battery to maintain its state of charge (SOC). [IMG=Simplified Diagram of Parallel Hybrid Drive System]/uploadpic/THESIS/2007/12/2007122416170823523F.jpg[/IMG] Figure 1 Simplified Diagram of Parallel Hybrid Drive System Electronic Control System Structure Design The electronic control system uses the internationally advanced CAN bus as the communication medium, with an intelligent multi-energy management unit as the control core, and five relatively independent intelligent nodes (front cabin sensor unit, interior sensor unit, motor drive unit, battery management unit, and display unit) as auxiliary nodes to form a network control system. The system's structural block diagram is shown in Figure 2. The multi-energy system obtains the current system status information through the CAN bus and generates control commands based on this status information. These commands are also sent to the engine control unit and motor drive unit through the bus to enable the two power sources to cooperate, thereby achieving the energy-saving and environmental protection goals of hybrid electric vehicles. The unified numbering of each unit (or node) in this design is as follows: N1 is the multi-energy control unit; N2 is the motor drive unit; N3 is the front cabin sensor unit; N4 is the interior sensor unit; N5 is the battery management unit; and N6 is the display unit. [IMG=Electronic Control System Structure Diagram 1]/uploadpic/THESIS/2007/12/2007122416171489260R.jpg[/IMG] Figure 2 Electronic Control System Structure Diagram 1 1) The multi-energy management unit is the command center of the entire system. It is composed of a high-performance microprocessor as its core, along with a large-capacity program and data memory and a bus interface. This allows it to quickly process system information transmitted from the bus. The large-capacity data memory stores the optimal operating parameters of the system, enabling the processor to promptly and accurately coordinate the two power sources—the motor and the engine—according to the control strategy. When a fault occurs in the electronic control system, it will promptly handle the fault to ensure the safe operation of the system. 2) The engine control unit consists of a microprocessor, program and data memory, D/A conversion circuit, switching interface, and bus interface. Its function is to receive commands to the engine from the multi-energy management unit via the bus, process and judge them, and then control the engine through analog and digital outputs. This mainly includes controlling the engine's air-fuel ratio and ignition system under various operating conditions. In addition, this unit will promptly report the execution status and its current condition (normal or faulty) to the multi-energy management unit via the bus. 3) The motor drive control unit consists of a microprocessor, program and data memory, D/A converter, digital interface, and motor speed control. Its function is to receive and execute control commands to the motor from the multi-energy management unit via the bus. It mainly controls the switching between generator and motor states, the motor speed, and the output torque. It reports the motor's status, such as speed, charging current, discharging current, and fault status, to the multi-energy management unit via the bus. When a motor fault occurs, it can perform self-processing to ensure the safe operation of the vehicle. 4) The data acquisition unit consists of a processor, microprocessor, program and data memory, digital interface, A/D converter, frequency converter, and bus interface. Its function is to accurately and promptly detect various parameters of the vehicle system, such as engine speed, vehicle speed, throttle opening, brakes, coolant temperature, vacuum, gear position, air conditioning status, key status, clutch status, etc., and transmit them to the multi-energy management unit via the bus, which uses this data as the basis for decision-making. In addition, this data also serves as the basis for the display unit to display information; therefore, this unit is the eye of the entire system. 5) Battery Management Unit: This hybrid vehicle uses a lithium battery pack consisting of 40 batteries connected in series. Each battery operates at a normal voltage of 3.6V. To ensure system safety and reliable battery operation, each battery is equipped with a controller based on a relatively simple microprocessor. Each controller monitors the voltage, capacity, and temperature parameters of the battery it manages and notifies the battery management unit via RS485 bus. Simultaneously, the controller also controls the charging and discharging current of the managed batteries to prevent overcharging and over-discharging, thus ensuring system safety. The battery management unit consists of a more advanced microprocessor and necessary peripheral circuitry. This unit communicates with the controllers of each battery via RS485 bus, collects the current status parameters (voltage, capacity, temperature) of each battery, processes this information, and notifies the multi-energy management unit via CAN bus. It also transmits the battery pack's SOC to the display unit for display. 6) Display Unit: The structure of the display unit is similar to that of other units. Its function is to receive information from the CAN bus and display necessary information such as vehicle speed, engine speed, mileage, battery capacity, charging current, discharging current, water temperature, fuel level, and system fault codes. In terms of display method, a high-precision digital display without moving parts is adopted, which increases the amount of information displayed and improves reliability. At the same time, in order to meet the driver's habit of reading pointer-type analog instruments, LEDs with pointer-like movement are used to simulate the display of old-style instruments. Hardware Design Considering the functional requirements of nodes N3, N4, and N6, designing the hardware separately according to function would allow for the selection of each node's needs, reducing node size and weight. However, this would hinder system expansion and limit node reliability and interchangeability. Therefore, a unified hardware design for the three nodes is adopted, meaning the hardware of all three nodes is identical and interchangeable. Each hardware component includes all the necessary functions for all three nodes. Furthermore, due to advancements in modern electronics, the difference in size and weight between the unified and separately designed hardware is minimal, having a negligible impact on system performance. More importantly, this design lays the hardware foundation for subsequent system expansion and redundant reliability design required for further improvements. The hardware system (Figure 3) is composed of a microprocessor CPU as its core and necessary peripheral circuits. The CPU is a high-performance central processing unit with rich functionality and high-speed processing capabilities, capable of rapidly performing numerous mathematical and logical operations, fully meeting the design requirements of this electronic control system. The program memory stores the software that implements the functions of each node to control its functionality. The data storage circuit is used for system memory expansion. Complex program control and calculations require a large number of intermediate variables and buffers, and memory expansion solves this problem. The D/A conversion circuit is used to convert the processed digital signals into analog signals for output. It is mainly used for output control and remote signal conversion. The A/D conversion circuit is used to acquire analog quantities of the system, that is, to convert the acquired analog signals into digital signals. The designed conversion circuit has a conversion speed of up to 5 microseconds/channel and a resolution of 1/4096. Each node can perform 8-channel A/D conversion simultaneously, which fully meets the technical requirements of the task specification. The switch output interface is used to output the bit control commands of the nodes. It has flexible output control and can directly or indirectly meet the control requirements of various power outputs. The main bit control signals in this system include engine fuel supply, air conditioning, and large and small motor start selection signals. The digital input circuit is used to acquire the system's digital inputs, such as key switch information, clutch status information, and air conditioning status information. The display interface circuit transmits the signals to the display in real time. The display uses a high-brightness digital LED display, improving display accuracy while retaining the original instrument panel layout, allowing users a smooth transition and preventing any unfamiliarity. The frequency conversion circuit converts the system's speed signals (engine speed and vehicle speed) into a standardized pulse signal for microprocessor recognition and speed measurement. The CAN bus circuit consists of a bus controller and transceiver control circuits. It is responsible for receiving and sending bus information, connecting nodes via the bus, making the electronic control system a flexible network control system. [IMG=Hardware System Structure Diagram]/uploadpic/THESIS/2007/12/2007122416172341634S.jpg[/IMG] Figure 3 Hardware System Structure Diagram Software Design Software Design of the Front Compartment Sensor Node (N3) The tasks to be completed by the N3 node are: 1) Real-time detection of parameters such as vehicle speed, engine speed, throttle position, coolant temperature, and vacuum; 2) Detection of faults in this node; 3) Completion of bus data transmission according to the communication protocol. Most tasks of this node require high real-time performance. Therefore, the software strategy of this node is: 1) Use interrupt mode to handle vehicle speed, engine speed, and bus communication; 2) Start A/D conversion periodically. To meet the real-time requirements of the tasks, the timing interval is 20μs to ensure timely digital measurement of throttle position, coolant temperature, and vacuum; 3) Query the system status during idle time to detect whether the system is normal. If there is a fault, handle the fault accordingly. This approach allows for real-time processing of vehicle speed, RPM, and communication interruptions, while also enabling the CPU to operate in a multitasking state, thus improving system resource utilization. The software design of the indoor sensor node (N4) involves the following tasks: 1) Real-time detection of key switch status, clutch status, air conditioning status, and brake parameter values; 2) Detection of faults within this node; 3) Completion of bus data transmission according to the communication protocol. The software strategy for this node is: 1) Interrupt-based processing of key switch status, clutch status, air conditioning status, and other switch quantities, as well as bus communication; 2) Timed initiation of A/D conversion, with a timing interval of 20μs to meet real-time requirements for timely digital measurement of brake signals; 3) Querying system status during idle time to check for system normality, and handling faults accordingly if any are found. The software design of the display node (N6) involves the following tasks: 1) Real-time display of parameters such as vehicle speed, mileage, charging current, discharging current, and battery capacity; 2) Timely control of the start-up of large and small motors and engine fuel supply; 3) Detection of faults within this node; 4) Completion of bus data transmission according to the communication protocol. The software strategy for this node is as follows: 1) Interrupt-based processing is used for bus communication, starting control of large and small motors, and fuel supply control of the engine to ensure timely control and communication; 2) The display is refreshed periodically, with a refresh interval of 20ms to meet real-time requirements; 3) The system status is queried during idle time to check if the system is normal. If a fault is found, it is handled accordingly. Communication Mechanism Design: A large amount of data transmission between units in this vehicle is accomplished through the CAN bus. This is a unique feature of this hybrid power electronic control system. The use of the CAN bus reduces the weight of the wiring harness in the electronic control system and lowers the system complexity. Furthermore, since the CAN bus is a differential transmission fieldbus with strong anti-interference capabilities, the reliability of system communication is ensured. CAN Bus Introduction: The CAN bus is a serial data communication protocol developed by Bosch in the early 1980s to solve the data exchange problem between numerous control and testing instruments in modern automobiles. CAN has become an international standard (ISO-11898) and is a fieldbus with international standards, specifications 2.0A and 2.0B. The CAN bus supports 8/16-bit CPUs and can interface with various processors or form intelligent instruments. It can operate in multi-master mode, allowing any node to actively send information at any time, without master-slave distinctions. This flexible communication method facilitates the construction of multi-machine fault-tolerant systems. Nodes can be assigned different priorities to meet various real-time requirements. Employing non-destructive bus arbitration technology, when multiple points transmit simultaneously, lower-priority nodes actively stop transmitting, while higher-priority nodes continue transmitting unaffected, effectively avoiding bus conflicts. It can transmit and receive data using point-to-point, point-to-multipoint, and global broadcast methods, with direct transmission distances reaching 10km/5Kbps. The system boasts a maximum data rate of 1Mbps/40m, with a theoretical maximum of 2000 nodes on the bus, but in practice, due to latency, this can be reduced to 110 nodes. It employs a short frame structure with 8 effective bytes per frame, resulting in short transmission time, low interference probability, and fast retransmission. Communication media can include twisted-pair cable and fiber optic cable. The user interface is simple and easy to program. It operates in temperatures ranging from -40℃ to +125℃. In case of node failure, it automatically shuts down the bus, allowing disconnection without affecting bus operation. Each frame includes CRC checksum and other detection measures to ensure an extremely low error rate. It exhibits high adaptability. The interface transceiver features instantaneous voltage protection, RT suppression, thermal protection, and short-circuit protection. Communication Protocol This system defines its communication protocol based on the CAN2.0A protocol. Node N1 sends a command, and nodes N2 through N6 receive it but do not send an acknowledgment signal. After receiving the information from nodes N2 through N6, node N1 checks its correctness. If it is incorrect or does not receive frames from N2 through N6 within the specified time, it resends the command. Exceeding the specified number of retransmissions constitutes a communication failure. Similarly, if nodes N2 through N6 send information, and node N1 receives it but does not send an acknowledgment signal, and nodes N2 through N6 do not receive the command from node N1 within the specified time, it also constitutes a communication failure. When node N6 receives the first frame after power-on reset from node N1, it responds within the specified time. If node N6 is functioning correctly during operation, it does not respond to node N1. The basic structure of node data frames is defined as follows: Each node data frame in the system is distinguished by an ID. Each node can define multiple different data frames to transmit different information. [IMG=Different Data Frames]/uploadpic/THESIS/2007/12/20071224161731439523.jpg[/IMG] Performance Analysis Table 1 compares the performance of hybrid electric vehicles (HEVs) and traditional electric vehicles (EVs). The test parameters show that HEVs are superior to EVs in terms of power and economy, and their emissions meet the Euro 2 standard. Furthermore, tests show that the amount of CO and NOx produced by incomplete combustion in their exhaust is less than that of EVs. [IMG=Performance Comparison of Hybrid Electric Vehicles and Traditional Electric Vehicles]/uploadpic/THESIS/2007/12/2007122416173754971C.jpg[/IMG] Conclusion A hybrid electric vehicle electronic control system consisting of intelligent nodes and a CAN bus communication network was proposed. In domestic hybrid electric vehicle control systems, a CAN bus network control system was used, and the excellent performance of the CAN bus was verified. From a performance perspective, under the excellent control strategies of the electronic control system, the engine of a hybrid electric vehicle can operate at maximum efficiency and low emissions, while also recovering some energy, achieving the energy-saving and environmental protection goals of hybrid power. Furthermore, the addition of electric motor power also improves the vehicle's dynamic performance. It can be concluded that with the further development of electronic control system technology, hybrid electric vehicles will inevitably become a hot topic in the green vehicle industry.