Design of Bus ABS Controller Based on MC9S12DP256B

The anti-lock Brake System (ABS) is used to prevent the wheels from being completely locked during the braking process of the car, improve the directional stability and steering ability of the car during braking, and shorten the braking. distance. For the automotive anti-lock braking system (ABS), the internationally popular control methods include logic threshold control, PID control, sliding mode variable structure control, optimal control, fuzzy control and neural network control. The control method adopted by ABS in China is mainly the most basic logic threshold control method. At present, there are many research institutes in China that study ABS theory. Some representative ones are: State Key Laboratory of Automotive Dynamic Simulation, represented by Academician Guo Konghui, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, South China Institute of Technology and Transportation Automobile Department, Jinan Chengjun Electronic Technology Company, etc. There are many domestic ABS companies, but most of them are produced in cooperation with famous foreign ABS companies. Their products are not independently developed. The more representative domestic companies that have completely developed ABS are Chongqing Juoneng Automotive Technology Co., Ltd. and Xi'an Bohua Electromechanical Co., Ltd. [1].

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During the braking process of the car, the wheel may slip relative to the road surface, and the proportion of the slip component in the longitudinal movement of the wheel may be characterized by the slip ratio. Define the slip ratio as s:

Where: v is the body speed, ω is the wheel angular velocity, and r is the wheel radius. The adhesion coefficient is also different at different slip ratios. Tests have shown that the relationship between longitudinal and lateral adhesion coefficient and slip ratio is shown in Figure 1.

The working principle of the anti-lock brake system is to control the slip ratio of the wheel near the optimal slip ratio sc to obtain a high longitudinal and lateral adhesion coefficient to reduce the braking distance and ensure the braking of the car. Directional stability.

1 ABS hardware design

1.1 MC9S12DP256B MCU [2]

The MC9S12DP256B is a member of the Motorola 16-bit microcontroller HCS12 family, and its processing unit uses a 16-bit STAR12 CPU. On-chip resources include 256KB of Flash ROM, 12KB of RAM, 4KB of EEPROM, an 8-channel Pulse Width Modulation Module (PWM), an 8-channel Enhanced Capture Timer Module (ECT), and two 8-channel A/ D conversion module (ATD), two serial communication interfaces (SCI), three serial device interfaces (SPI), etc. In the Codewarrior integrated development environment, the program can be edited, compiled, downloaded and debugged online, making it easy to develop.

The ECT module has eight input capture/output compare (IC/OC) channels, four 8-bit or two 16-bit pulse accumulator (PAI) channels. When the module is running, the 16-bit free timer loops between $0000 and $FFFF at the set clock frequency. If a channel is set to I/O function, when the set edge of the signal under test comes, the input capture logic immediately captures the contents of the free timer into the 16-bit IC/OC register with a resolution of up to 1μs or more. High, and set the interrupt request flag, and then the program performs interrupt processing. If a channel is set to the OC function, the output compare logic automatically compares the contents of the IC/OC register with the contents of the free timer. Once the match, the corresponding pin is immediately operated, and the corresponding interrupt flag is set, and then the program performs interrupt processing. The time resolution of the pin output waveform can also be as high as 1μs or higher. The pulse counter counts only the number of inputs or edges and does not generate an interrupt. This function is used in the wheel speed acquisition algorithm.

The IC/OC shares eight pins with the general purpose I/O port PORTT. The four 8-bit PAI channels 0 to 3 share the pins PORTT0~3 with the first four IC channels IC0~3. In this controller, PORTT0~3 uses the pulse accumulator function to record the number of pulses of the four wheel speed sensors. When a real-time interrupt (RTI) is generated, the interrupt program reads the value of the pulse accumulator, calculates the speed of the wheel, and simultaneously clears the pulse accumulator to restart counting. PORTT4~7 uses the output compare function. When the value of the IC/OC register is equal to the value of the free counter, an interrupt is generated. The four interrupt programs process the ABS control strategies of the respective wheels.

1.2 Electronic Control Unit ECU Hardware Structure

The core component of ABS is the Electronic Control Unit (ECU). The ECU circuit mainly includes four modules: power module, wheel speed signal processing module, arithmetic module, and solenoid valve drive circuit. Its basic function is to realize wheel speed acquisition, ABS fault detection, and control signal to solenoid valve according to control law. . Its structural block diagram is shown in Figure 2.

2 ABS control algorithm

The ABS control algorithm uses logic threshold control. The basic principle is that the acceleration and deceleration of the wheel is used as the main control threshold, and the slip rate of the wheel is used as the auxiliary control threshold. When the deceleration reaches the lower threshold, the decompression control signal is issued, and the acceleration reaches the first upper threshold. The boost control signal is issued. This cycle is repeated until the wheel speed drops to a lower value, exiting the ABS control.

Different road surface adhesion factors use different control strategies, so the first step in the algorithm is to identify the road surface. The method of road surface recognition is as follows: firstly, a pressure maintaining signal is issued to the wheel, and after a certain period of pressure, the road surface is identified according to the wheel reduction speed at this time. If the deceleration rate at this time exceeds the second upper threshold, it indicates that the road is highly attached; if the deceleration is between the first upper threshold and the second upper threshold, it indicates that the road surface is generally attached; if the wheel is reduced If the speed is less than the first upper threshold, it means that the road is low. The result of this is that the holding time of each wheel occupies most of the cycle time. If the cycle sequence of four wheels is used, it takes a long time to meet the timeliness requirement. However, if a multi-tasking real-time operating system is introduced, the algorithm is too complicated. Therefore, a method of introducing four interrupt processing is proposed, that is, each wheel has an independent timing clock, and the ABS control is executed in accordance with the respective set interruption time, realizing parallel control of four wheels. The interrupt time is set to ensure that the ABS control loop is executed ten to dozens of times in one second.

2.1 ABS detailed control strategy

Step 1: When the deceleration speed reaches the lower threshold, the system starts to maintain pressure and calculates the slip ratio until it is judged that the wheel enters the unstable area. Set the stage flag to Flag=2 and decompress.

Step 2: Determine the deceleration until the deceleration is less than the lower threshold, set Flag=3, hold pressure.

Step 3: Hold pressure for a certain period of time and judge the magnitude of the acceleration at this time: (1) If the acceleration is less than the first upper threshold, it is judged to be low adhesion to the road surface, set Flag=41, decompress. (2) If the acceleration is between the first upper threshold and the second upper threshold, it is judged to be a general road surface, and the flag is set to 42 and the pressure is maintained. (3) If the acceleration is greater than the second upper threshold, it is judged to be a high adhesion road surface, set Flag=43, and boost.

Step 4: Perform different control schemes according to the Flag value obtained by Step3.

Step 5: After one phase is completed, set Flag=1 to prepare to enter the next cycle.

Each time the program enters the interrupt, it will perform different control phases according to the value of the phase flag until the entire ABS control is completed.

2.2 Wheel speed processing algorithm [4]

Wheel speed is the basis for calculating wheel acceleration and deceleration in the ABS program. The treatment of the wheel speed must satisfy: (1) good real-time performance. ABS's anti-lock control requires multiple cycles in a second, so the timeliness of wheel speed processing is very high, and the wheel speed processing program is not too complicated. (2) High precision. The accuracy of the ABS wheel speed has a great influence on the accuracy of the subsequent wheel acceleration and deceleration and the reference vehicle speed.

The method of wheel speed acquisition usually has a periodic method and a pulse counting method, and a pulse counting method is used here. The pulse counting method calculates the wheel speed by using the number of ring gears collected by the wheel speed sensor for a certain period of time, that is, the number of pulses. Its calculation formula is:

ω=2(πr/N)×(n/Δt) (2)

Where ω is the wheel angular velocity; r is the wheel radius; N is the number of ring gear teeth; n is the number of recorded pulses; Δt is the measurement time interval. It can be seen from equation (2) that the calculation error is mainly caused by the latter half. The timing of the MCU is very accurate, so the error of Δt can be neglected, but the number of pulses n is easy to cause the measurement error of ±1 tooth. The error generation diagram is shown in Figure 3.

At low speeds, this ±1 tooth error is likely to cause ABS malfunction. If the measurement time interval Δt is increased, the ABS control aging is deteriorated. Here, the average method is used, that is, the Δt is kept constant, and the average of the pulse count values ​​of the last four times is calculated for each round of the wheel speed, thereby reducing the random error, which can reflect the change trend of the wheel deceleration, and It can prevent ±1 tooth error from causing large interference to the calculation.

2.3 Program flow

The program flow chart is shown in Figure 4, Figure 5, and Figure 6, respectively.

3 simulation platform and results

The simulation mode uses the xPC Target structure. xPC Target is a complementary product of the RTW (Real-Time Workshop) framework released by MathWorks, which converts Intel 80x86/Pentium computers or PC compatible machines into a real-time system and supports many types of I/O interface boards. The "dual-machine" solution of the host and target machines uses two PCs, where the host is used to run Simulink and the target machine is used to execute real-time code. The target machine runs a highly compact real-time operating kernel that communicates between the host and the target via an Ethernet connection. After the simulation is completed, the result data can be uploaded to the host for analysis and processing.

The whole vehicle model of the bus is built in the Matlab/Simulink environment of the host machine, and then the model is automatically converted into C code by using the xPC tool, and downloaded into the industrial computer through the Ethernet as the controlled object to realize real-time simulation. Signal reception and conversion in the hardware section was done using Advantech's data acquisition card PCL-726. The controller's development platform uses Metrowerks' Codewarrior 3.1, which is compiled and downloaded to the controller (ECU) and debugged in BDM mode. The semi-physical simulation platform is shown in Figure 7.

The simulation results of one of the wheels are shown in Figure 8.

Fig. 8 is a single-round result diagram of the vehicle initial velocity of 25 mps and the road surface adhesion coefficient of 0.6. The four curves in the figure represent body speed, wheel speed, solenoid signal and brake pedal signal. The solenoid valve high state indicates boost pressure, the 0 state indicates holding pressure, and the low state indicates pressure reduction. After 1 second, the brake signal is given and the ABS starts to function. It can be seen from the figure that the burr is large in the place where the wheel speed is suddenly reduced, and the solenoid valve is in the stage of decompression or subsistence pulse; in the place where the wheel speed changes relatively gently, it is in the stage of supercharging or increasing the pulse. After 9 seconds, the body speed dropped below 3mps, automatically exiting the ABS control and returning to the normal brake. There was no wheel lock in the entire braking phase, and the result was ideal.

This paper introduces the design of passenger car ABS controller by using Motorola MC9S12DP256B, and proposes the use of four-level interrupt to realize parallel task processing. The simulation platform uses the rapid prototyping technology, and uses the xPC Target tool that comes with Matlab to directly generate the executable C code from the Simulink model to form a closed-loop semi-physical simulation platform. After simulating various working conditions, the controller has achieved good control effects and can be further optimized by real vehicle testing to achieve the finished product.

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