System Introduction The electrohydraulic power steering system (EHPS) consists of the following components: the power steering control unit (ECU), the power steering sensor, the gear pump with electric pump, the oil storage tank, the steering gear, and the CAN bus system. Our main job is to assist in the research and development of hardware and software for steering control units (ECUs). The power steering control unit is mainly based on the signal transmitted from the power steering sensor, and the speed of the electric pump is controlled by considering the vehicle speed on the CAN bus, thereby controlling the pumping amount of the gear pump to achieve the purpose of controlling the power steering transmission device. In this way we can determine the amount of steering assistance we currently need based on the steering angular speed of the vehicle's steering wheel and the current speed of the vehicle.
1. ECU design summary
1.1 ECU CPU selection controller (ECU) design key is to choose a single microcontroller for a specific control target, for the vehicle electro-hydraulic power steering system, it has the following characteristics for the microcontroller:
1 Anti-environment interference, meeting the temperature range of automotive grade chip, namely -40~+125°C.
2 There are more I/O ports to achieve various control functions; fast calculation speed to meet real-time control requirements.
3 The standard communication interface is adopted to facilitate the realization of the diagnostic function and the ability to communicate with other CPUs on the vehicle.
4 It has a good performance-price ratio and is convenient for mass loading, making it a popular product.
Based on the above characteristics, the 16-bit single-chip microcomputer is a moderate choice for the electro-hydraulic power steering system. And most of the foreign electro-hydraulic power steering system products currently use this grade of single-chip microcomputer. The Infineon XC164CM is this mid-range product, a 16-bit microcontroller designed by Infineon for vehicle control systems.
1.2 ECU module design 
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See Figure 1. The design is based on the Infineon XC164CM microcontroller, which includes four modules: input, output, fault diagnosis, and vehicle information data transmission.
1 input module input adopts XC164CM's input capture function CAPCOM unit, and each input channel corresponds to a separate CAPCOM channel. The CAPCOM channel can issue an interrupt after each event is received. It has a capture register, CCx, which holds one event. Since the interrupt address is independent, it is no longer necessary to identify which interrupt source is entered into the interrupt service routine, which improves the efficiency of the interrupt service processing. In this system, CC16 and CC17 are also used as the input signal capture channels of the two Hall device pulses, and CC23 is used as the input signal capture channel for the steering wheel angle pulse. The CAPCOM channel is used to greatly improve the quality of the acquired signal, and the CPU takes up less time. The resolution of the capture event of CAPCOM is 400 ns at 20MHz.
2 output module
The XC164CM has 47 I/O ports, and the P1 port is used as the control output port in this system. Each I/O port uses a separate controller to control the input/output.
3 diagnostic module
EHPS is a system with high reliability requirements. It requires internal programs to monitor the system at any time. Once a fault occurs, it can be recorded in time and stored. This system uses E2 PROM of I2C bus as fault code memory, realizes I2C bus and E2PROM communication with XC164CM synchronous serial function, XC164CM as the host and E2PROM as the slave. The external diagnostic communication is realized through the CAN1 port of the XC164CM, and the communication is connected with the PC through the USB data acquisition card.
4 Vehicle information data transmission MCU XC164CM with two CAN bus controllers, CAN1 and CAN2. There is 2K bytes of CAN register space for CAN communication. It has 32 information bodies, and each message body has 8 bytes of data to be transmitted. The speed and engine speed required for the EHPS system is 2. ECU software design The program structure adopts the multi-module structure popular in C language. This structure has clear program structure compared with the previous single-module structure, which is convenient for program migration and program maintenance.
2.1 Main program of EHPS ECU
The EHPS main loop is a timed loop caller, and the block diagram is shown in Figure 2. 
It mainly calls each subroutine. Timing time depends on the complexity of the algorithm. When the algorithm is complex and time consuming, the time of each cycle can be defined longer. If the CPU is more powerful and the algorithm is simpler, a shorter cycle time can be used. For example, 5ms or even less. Some time-consuming programs, such as diagnostics, can be placed outside the EHPS main loop. The initialization program is called each time the program starts executing. First, the hardware peripherals are initialized, such as serial port and timer. These initializations are defined only once per power-up and open interrupts, then software initialization is required to set the EHPS control threshold. Some of these thresholds change during the control process, and some are constant constants. If it is a constant constant, it can be defined by the method defined by the macro in C language. Calling the diagnostics is a comprehensive diagnosis of the system. This program takes a long time, so it is only outside the EHPS main loop. Once the EHPS system is ready, it enters the EHPS loop. The EHPS cycle can generally be defined as between 4 and 50 ms. Calling various algorithms between this time, to ensure that each program runtime adds up to less than the EHPS cycle time, in order to guarantee the rhythm of EHPS. If the program execution time is longer than the EHPS cycle time, the program work will not be strong, which will cause the system to run unstable. The EHPS cycle time is adjustable and is adjusted based on the execution time of the program.
2.2 EHPS ECU interrupt program
The EHPS main loop is interrupted by the interrupt system from time to time. Once an interrupt occurs, the CPU responds to the interrupt and exits the main loop. The service program of the interrupt system mainly handles bursts and random events. If there is no interruption, it will take up a lot of query time of the main program, so the interrupt system is actually a parallel multitasking program.
The following types of interrupts are mainly used in the EHPS software:
1) Steering wheel corner interruption: It uses the input capture interrupt to record the steering wheel angle pulse signal to the register for calculating the target speed.
2) Motor speed acquisition interrupt: It can record the pulse signal of the input speed to the register at any time by using the input capture interrupt.
3) Serial communication interruption: Serial communication is mainly used for the diagnosis system. It uses the receiving interrupt. When receiving the signal from the upper computer, it can interrupt the main loop and enter the diagnostic program.
4) Timer interrupt: Perform delay processing of the control system.
3. ECU Hardware Circuit System Design The electronic control unit determines and controls the motor speed and drive current based on the steering rate and vehicle speed signals. It optimizes the steering assist torque at every speed. In order to protect the power drive circuit, the motor current needs to be sampled. In order to reduce the acquisition noise, a first-stage sample-and-hold circuit is used for the system current input, and the differential input terminals are all passed through the sample-and-holder. The voltage is sampled by the CC20 and CC21 of the P9 port. sampling. The motor current differential amplifier circuit principle is shown in Figure 3. In order to improve the system's anti-interference ability, a 5mV bias voltage is applied to the input terminal. In order to control the current commutation time of the motor, two Hall speed sensors are used to precisely control the commutation of the motor. The Hall speed sensor can be used with the TLE4905. In order to control the speed of the motor, PWM chopper is used to control the motor voltage. Use the P1L0~P1L3 of the P1 port of the XC164CM to control the phase of the motor, and use the CC24 and CC25 of the P1 port to chop the voltage. The waveform is superimposed by the 74LS366 and sent to the power MOSFET. 
4. Comparison of test results
We used MATLAB/Simulink control development and test platform to simulate our EHPS system. Figure 4 and Figure 5 show the comparison between the self-developed ECU and the ECU control effect on the imported original car. The control effects of the two controllers are consistent. At the same time, we carried out the driving experience of different drivers. Without knowing which controller is not in advance, the driver can not tell whether the ECU on the original car or the self-developed ECU is working.
Conclusion In the nearly two years of research, we mainly used on-line debugging of control programs on real vehicles, and repeated performance comparison with similar foreign products. This ensures the feasibility of the product in real vehicle application. According to the motor control curve of similar foreign products, our EHPS ECU has basically reached the level of similar foreign products in terms of performance. However, reliability should be further verified as the product continues to be loaded.
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