Design of fuel cell electric vehicle controller based on MC9S12DP256

The vehicle control unit (VCU) of a fuel cell electric vehicle is the core control unit of the entire vehicle. It collects signals from the accelerator pedal, brake pedal and other components, and after making corresponding judgments, it controls the actions of the lower-level component controllers to drive the vehicle to run normally. Therefore, the quality of the VCU directly affects the performance of the entire vehicle.

The development of fuel cell electric vehicle controller is one of the key unit technologies of the national electric vehicle major project during the "15th Five-Year Plan". The basic research of these key unit technologies is of great significance for seizing the commanding heights of the new generation of electric vehicles and promoting the leapfrog development of my country's automobile industry.

1 Fuel Cell Electric Vehicle Structure

The structural block diagram of a fuel cell electric vehicle is shown in Figure 1. It consists of a vehicle controller, a fuel cell and its controller, a nickel-hydrogen battery pack and its controller, a drive system, wheels and other components. Each component forms a distributed control system through the CAN (Controller Area Network) bus. Fuel cell electric vehicles use a main and auxiliary dual power source: the fuel cell is the main power source, providing the main power for the vehicle to drive; the nickel-hydrogen battery pack is an auxiliary power source, which plays the role of "peak shaving and valley filling" during vehicle driving.

2. Vehicle controller hardware function circuit design

2.1 Analysis of vehicle controller functional requirements

The vehicle controller is equivalent to the brain of the car. It performs multiple tasks during the driving process. The specific functions include: (1) receiving and processing the driver's driving operation instructions, and sending control instructions to the controllers of various components to make the vehicle drive as expected. (2) Reliable communication with the motor, DC/DC, nickel-metal hydride battery pack, etc., and collecting state input and output of control instruction quantity through the CAN bus (as well as the analog quantity of key information). (3) Receive and process information from various components, and provide current energy status information in combination with the energy management unit. (4) Determine and store system faults, dynamically detect system information, and record faults. (5) It has a protection function for the entire vehicle, and provides graded protection for the entire vehicle depending on the type of fault. In an emergency, the generator can be turned off and the bus high-voltage system can be cut off. (6) Coordinate and manage other electrical equipment on the vehicle.

In view of the specific functions of the vehicle controller, the overall hardware design planning, MCU selection and design of each functional circuit were carried out as shown in Figure 2.

2.2 MCU selection

MCU is the core of the vehicle controller, which is responsible for data collection and processing, logical operations and control implementation, etc. The selection of MCU is the most important task in the entire hardware design process. Motorola's HCS12 series 16-bit microcontroller MC9S12DP256 has superior performance in computing power, storage space, digital and analog input and output, and CAN communication, and has a high cost-effectiveness, making it very suitable for some mid-to-high-end automotive electronic control systems.

This single-chip microcomputer has many features such as strong budget capability, large storage space, and rich interface resources:
(1) It uses STAR12CPU, the core computing power can reach 50MHz, the bus speed can reach 25MHz, and the optimized instruction set is used to greatly improve the computing speed of instructions.
(2) The chip integrates 256KBFLASH, 12KBRAM and 4KBE2PROM, which can fully meet the program's requirements for storage space.
(3) Many external interfaces, including five CAN interfaces compatible with CAN2.0A/B protocol, two asynchronous serial communication interfaces, three synchronous serial communication interfaces, sixteen 10-bit A/D interfaces, one I2C bus interface, 49 independent digital I/O ports (20 of which have external interrupt and wake-up functions), 8-channel input capture/output comparison, etc. [page]

2.3 VCU Hardware Circuit Design

The vehicle controller is a complex system with multiple inputs, multiple outputs, and coexisting digital and analog circuits, and its various functional circuits are relatively independent. Therefore, the various modules of the hardware system are designed according to the modular concept, mainly including: minimum application system module, power module, CAN communication module, serial communication module, and digital and analog input and output module.

2.3.1 Power Module

The power supply of the vehicle controller comes from the nickel-hydrogen battery pack of the fuel cell electric vehicle, and its rated voltage is +12V. During the operation of the vehicle, the voltage of the nickel-hydrogen battery pack is unstable and fluctuates greatly. It can reach +17V at high voltage and only +9V at low voltage. The instability of the power supply voltage will directly lead to abnormal operation of the vehicle controller. Therefore, in the design process of the power module, a DC/DC power supply chip with a wide input range, high output accuracy and high power is adopted. In addition, since the power supply voltage of the chip used in the vehicle controller includes +5V and +12V, two DC/DC chips are used in the design: TLE4270 of Infineon and LM2940S-12 of National Semiconductor, which have the voltage conversion and voltage stabilization functions of 12V-12V and 12V-5V respectively, and have the functions of short circuit, overvoltage, overcurrent and temperature overload protection. By using these two chips and some peripheral auxiliary circuits (mainly filtering circuits), the power supply of the power module is stable and reliable.

2.3.2CAN communication module

Since the MC9S12DP256 chip integrates five CAN modules compatible with the CAN2.0A/B protocol, the CAN communication module of the vehicle controller does not need to add an external CAN controller, but only an external CAN transceiver. The designed CAN communication module uses the TJA1040 transceiver chip of PHILIP. The baud rate range of this chip is 60kbps~1Mbps. It has a temperature protection circuit that disconnects the transmitter when the temperature of the connection point with the transmitter exceeds about 165℃ (this temperature protection circuit is even more necessary when the bus is short-circuited) [2].

In order to enhance the anti-interference ability of the CAN bus node, the CANTXD and CANRXD pins of the main chip are not directly connected to the TXD and RXD pins of the TJA1040, but are connected to the TJA1040 through the high-speed optical coupler HCPL-0630. In this way, when there are multiple CAN nodes on the bus, electrical isolation between the CAN nodes can be achieved. The interface between TJA1040 and the CAN bus also takes certain safety and anti-interference measures:
(1) The CANH and CANL pins of TJA1040 are each connected to the CAN bus through a 5Ω resistor. The resistor can play a certain current limiting role and protect TJA1040 from overcurrent shock.
(2) Two 30pF small capacitors are connected in parallel between CANH and CANL and the ground, which can filter out high-frequency interference on the bus and have a certain anti-electromagnetic radiation ability.
(3) A protection diode is reversely connected between the two CAN bus access ends and the ground. When the CAN bus has a high negative voltage, the short circuit of the diode can play a certain overvoltage protection role.

2.3.3 Digital-to-analog input and output modules

During the operation of fuel cell electric vehicles, the vehicle controller often sends signals such as vehicle start/stop and nickel-metal hydride battery pack closing/disconnecting, i.e., digital output. To ensure stable and reliable signals, the vehicle controller is equipped with four digital outputs, all of which are greater than 50mA. The design uses relay-based switch output, which is currently the most commonly used output method. The relay chip used is Infineon's BTS824R, which has the following features [3]:
(1) Wide voltage range input, compatible with CMOS and TTL levels.
(2) Enhanced electromagnetic compatibility design.
(3) Built-in short circuit protection, overload protection, and ESD protection.
(4) Built-in over-temperature cut-off protection.

The vehicle controller sends out on and off signals while receiving corresponding digital signals. A high-speed optocoupler HCPL-0630 is used between the main chip MC9S12DP256 and the external signal to achieve level conversion and signal isolation.

The vehicle control unit (VCU) of a fuel cell electric vehicle is the core control unit of the entire vehicle. It collects signals from the accelerator pedal, brake pedal and other components, and after making corresponding judgments, it controls the actions of the lower-level component controllers to drive the vehicle to run normally. Therefore, the quality of the VCU directly affects the performance of the entire vehicle.

The development of fuel cell electric vehicle controller is one of the key unit technologies of the national electric vehicle major project during the "15th Five-Year Plan". The basic research of these key unit technologies is of great significance for seizing the commanding heights of the new generation of electric vehicles and promoting the leapfrog development of my country's automobile industry.

1 Fuel Cell Electric Vehicle Structure

The structural block diagram of a fuel cell electric vehicle is shown in Figure 1. It consists of a vehicle controller, a fuel cell and its controller, a nickel-hydrogen battery pack and its controller, a drive system, wheels and other components. Each component forms a distributed control system through the CAN (Controller Area Network) bus. Fuel cell electric vehicles use a main and auxiliary dual power source: the fuel cell is the main power source, providing the main power for the vehicle to drive; the nickel-hydrogen battery pack is an auxiliary power source, which plays the role of "peak shaving and valley filling" during vehicle driving.

2. Vehicle controller hardware function circuit design

2.1 Analysis of vehicle controller functional requirements

The vehicle controller is equivalent to the brain of the car. It performs multiple tasks during the driving process. The specific functions include: (1) receiving and processing the driver's driving operation instructions, and sending control instructions to the controllers of various components to make the vehicle drive as expected. (2) Reliable communication with the motor, DC/DC, nickel-metal hydride battery pack, etc., and collecting state input and output of control instruction quantity through the CAN bus (as well as the analog quantity of key information). (3) Receive and process information from various components, and provide current energy status information in combination with the energy management unit. (4) Determine and store system faults, dynamically detect system information, and record faults. (5) It has a protection function for the entire vehicle, and provides graded protection for the entire vehicle depending on the type of fault. In an emergency, the generator can be turned off and the bus high-voltage system can be cut off. (6) Coordinate and manage other electrical equipment on the vehicle.

In view of the specific functions of the vehicle controller, the overall hardware design planning, MCU selection and design of each functional circuit were carried out as shown in Figure 2.

2.2 MCU selection

MCU is the core of the vehicle controller, which is responsible for data collection and processing, logical operations and control implementation, etc. The selection of MCU is the most important task in the entire hardware design process. Motorola's HCS12 series 16-bit microcontroller MC9S12DP256 has superior performance in computing power, storage space, digital and analog input and output, and CAN communication, and has a high cost-effectiveness, making it very suitable for some mid-to-high-end automotive electronic control systems.

This single-chip microcomputer has many features such as strong budget capability, large storage space, and rich interface resources [1]:
(1) It uses STAR12CPU, the core computing power can reach 50MHz, the bus speed can reach 25MHz, and the optimized instruction set is used to greatly improve the instruction computing speed.
(2) It integrates 256KBFLASH, 12KBRAM and 4KBBE2PROM on the chip, which can fully meet the program's requirements for storage space.
(3) There are many external interfaces, including five CAN interfaces compatible with CAN2.0A/B protocol, two asynchronous serial communication interfaces, three synchronous serial communication interfaces, sixteen 10-bit A/D interfaces, one I2C bus interface, 49 independent digital I/O ports (20 of which have external interrupt and wake-up functions), 8-channel input capture/output comparison, etc.

2.3 VCU Hardware Circuit Design

The vehicle controller is a complex system with multiple inputs, multiple outputs, and coexisting digital and analog circuits, and its various functional circuits are relatively independent. Therefore, the various modules of the hardware system are designed according to the modular concept, mainly including: minimum application system module, power module, CAN communication module, serial communication module, and digital and analog input and output module.

2.3.1 Power Module

The power supply of the vehicle controller comes from the nickel-hydrogen battery pack of the fuel cell electric vehicle, and its rated voltage is +12V. During the operation of the vehicle, the voltage of the nickel-hydrogen battery pack is unstable and fluctuates greatly. It can reach +17V at high voltage and only +9V at low voltage. The instability of the power supply voltage will directly lead to abnormal operation of the vehicle controller. Therefore, in the design process of the power module, a DC/DC power supply chip with a wide input range, high output accuracy and high power is adopted. In addition, since the power supply voltage of the chip used in the vehicle controller includes +5V and +12V, two DC/DC chips are used in the design: TLE4270 of Infineon and LM2940S-12 of National Semiconductor, which have the voltage conversion and voltage stabilization functions of 12V-12V and 12V-5V respectively, and have the functions of short circuit, overvoltage, overcurrent and temperature overload protection. By using these two chips and some peripheral auxiliary circuits (mainly filtering circuits), the power supply of the power module is stable and reliable.

2.3.2CAN communication module

Since the MC9S12DP256 chip integrates five CAN modules compatible with the CAN2.0A/B protocol, the CAN communication module of the vehicle controller does not need to add an external CAN controller, but only an external CAN transceiver. The designed CAN communication module uses the TJA1040 transceiver chip of PHILIP. The baud rate range of this chip is 60kbps~1Mbps. It has a temperature protection circuit that disconnects the transmitter when the temperature of the connection point with the transmitter exceeds about 165℃ (this temperature protection circuit is even more necessary when the bus is short-circuited) [2].

In order to enhance the anti-interference ability of the CAN bus node, the CANTXD and CANRXD pins of the main chip are not directly connected to the TXD and RXD pins of the TJA1040, but are connected to the TJA1040 through the high-speed optical coupler HCPL-0630. In this way, when there are multiple CAN nodes on the bus, electrical isolation between the CAN nodes can be achieved. The interface between TJA1040 and the CAN bus also takes certain safety and anti-interference measures:
(1) The CANH and CANL pins of TJA1040 are each connected to the CAN bus through a 5Ω resistor. The resistor can play a certain current limiting role and protect TJA1040 from overcurrent shock.
(2) Two 30pF small capacitors are connected in parallel between CANH and CANL and the ground, which can filter out high-frequency interference on the bus and have a certain anti-electromagnetic radiation ability.
(3) A protection diode is reversely connected between the two CAN bus access ends and the ground. When the CAN bus has a high negative voltage, the short circuit of the diode can play a certain overvoltage protection role. [page]

2.3.3 Digital-to-analog input and output modules

During the operation of fuel cell electric vehicles, the vehicle controller often sends signals such as vehicle start/stop and nickel-metal hydride battery pack closing/disconnecting, i.e., digital output. To ensure stable and reliable signals, the vehicle controller is equipped with four digital outputs, all of which are greater than 50mA. The design uses relay-based switch output, which is currently the most commonly used output method. The relay chip used is Infineon's BTS824R, which has the following features [3]:
(1) Wide voltage range input, compatible with CMOS and TTL levels.
(2) Enhanced electromagnetic compatibility design.
(3) Built-in short circuit protection, overload protection, and ESD protection.
(4) Built-in over-temperature cut-off protection.

The vehicle controller sends out on and off signals while receiving corresponding digital signals. A high-speed optocoupler HCPL-0630 is used between the main chip MC9S12DP256 and the external signal to achieve level conversion and signal isolation.

3. Vehicle controller reliability design and testing

On the basis of perfect functions, reliability is the main technical indicator of the quality of the vehicle controller. In the working environment of the vehicle controller of fuel cell electric vehicles, the bus current transmitted by the motor, inverter and nickel-metal hydride battery pack varies greatly (especially when the inverter performs high-frequency modulation), and the spatial electromagnetic interference generated is very strong; in addition, the temperature variation range of its working space is wide and the vibration intensity is high. The above adverse factors may cause interference consequences to the vehicle controller mainly in the following aspects:
(1) Increased data acquisition error.
(2) Control state failure.
(3) Data changes due to interference.
(4) Program operation malfunctions.

In order to ensure the normal operation of the vehicle controller, this reliability design adopts a method that combines component-level reliability design and system-level reliability design, which is specifically reflected in: chip temperature range control, component redundancy design, system electromagnetic compatibility design, etc.

3.1 Chip temperature range

In the design of vehicle controllers, the temperature range of most chips is automotive grade (-40℃～+125℃), and a very small number of other chips choose industrial grade (-40℃～+85℃) due to price reasons.

3.2 Redundancy Design

Redundancy design refers to the technology of reducing the impact of failures by adding redundant resources to the system structure, or isolating and correcting the failures, so that even if a failure or error occurs in the system, its functions are still not affected [4]. This redundant design is achieved by increasing the number of functional circuits, and the overall redundancy reaches more than 50%, as shown in Table 1.

3.3 Electromagnetic compatibility design

Since the application environment of the vehicle controller is relatively harsh, interference is serious, and there are various noise and coupling modes, electromagnetic compatibility design occupies a very important position in all reliability designs. Anti-interference technologies such as filtering technology, decoupling circuit, shielding technology, isolation technology and grounding technology are adopted in the design [5] [6], as follows:
(1) Select components with high integration. It can reduce the number of components on the circuit board, make the circuit board layout simple, reduce pads and connections, and thus greatly reduce the probability of interference and increase the anti-interference ability of the circuit board.
(2) Thicken the power line and ground line, and keep the data line, address line and control line as short as possible to reduce the capacitance to the ground.
(3) The digital circuit and analog circuit are arranged in partitions, and filtering and decoupling circuits are added.
(4) Use a four-layer circuit board design. Compared with a two-layer board, there are independent ground planes and power planes, and the signal line and ground line can be very close, so it can effectively reduce common mode impedance and inductive coupling.
(5) Use copper plating technology. It can reduce the loop area (thereby reducing radiation) and reduce crosstalk between wires.

3.4 Reliability Test

The State Key Laboratory of Vehicle Dynamics Simulation of Jilin University conducted a preliminary reliability test on the vehicle controller developed and designed. The test process is as follows:
(1) High and low temperature test: maintain it at low temperature of -25℃ and high temperature of 125℃ for 6 hours respectively.
(2) Vibration test: scan frequency range 17～200Hz, maximum amplitude 0.78mm, acceleration 50 at 60～200Hz, and one scan time 15min.
(3) Electromagnetic compatibility test: use the real car to simply simulate various automotive electromagnetic interference conditions for preliminary testing.

During the entire test process, the vehicle controller worked normally without any reset phenomenon, and each functional module sent and received data normally. During the vibration test, no components fell off or were damaged.

4. Vehicle bench test

After the reliability test, the vehicle controller was connected with the fuel cell and its controller, the motor and its controller, the nickel-metal hydride battery pack and its controller, and other components to realize the powertrain test bench of the entire fuel cell electric vehicle. The following tests were performed on the bench:
(1) Communication joint debugging test: control system CAN communication test; data monitoring system signal acquisition.
(2) Vehicle controller control logic test: according to the same driving mode as the actual vehicle, focus on the single-mode debugging of the control logic of acceleration mode, start mode, charging mode, regenerative braking mode, power battery charging mode, and cruise driving mode.
(3) Vehicle controller control alarm test.
(4) Vehicle controller control mode switching test: focus on the switching between various control modes.

During the entire bench test, the vehicle controller ran stably, and each functional module completed its task according to the specified procedure, without any reset or data loss. Figure 3 is the pedal opening signal collected during the test, and the collected signal is continuous and complete. The vehicle controller not only achieved the established goals in terms of function, but also met the standards in terms of reliability.

The developed vehicle controller for fuel cell electric vehicles not only realizes the required functions, but also has good reliability and engineering practicality. The design of some important circuit modules and the method used in system reliability design have laid the foundation for the development of various electric vehicle controllers in the future.