Anti-skid control is an effective means to ensure the safety of railway vehicles and improve braking efficiency. Especially with the increase of freight speed, once the taxiing occurs, it may cause more harm. Therefore, anti-skid devices should be considered on the fast trucks in China [1]. Qiqi Equipment Co., Ltd. and Meishan Vehicle Co., Ltd. installed a mechanical anti-skid device on the prototype of the fast truck developed to solve the problem of wheel-to-slide. However, there is a mechanical non-slip low sensitivity, unstable, sticking state can not be monitored in real time, the brake pressure regulator disadvantages [1], and the mechanical key Antiskid sensitive element subject to wear long-term use, which results in decreased performance . Electronic anti-skid control using computer technology, real-time detection axles coasting condition, adjust the brake cylinder pressure to prevent slippage of the axle, capable of determining whether the sliding axle, with higher accuracy in accordance with multiple slip criterion, the railway passenger, EMU such as the rolling stock to be widely used.
The use of electronic anti-skid devices on fast trucks requires solving power problems. Southwest Jiaotong University has designed a wind power supply device that can be hung from the bottom of the car body [2]. Based on this, the paper studies the electronic anti-skid device for trucks, and proposes an anti-skid controller based on digital signal processor (DSP-TMS320F2812). The fuzzy control is used as the algorithm to realize the axle detection and anti-skid control of the axle. .
1 Fast truck electronic anti-skid principle
Due to the adhesive limit, the vehicle is prone to slippage and even wheel scratches during braking. In our country the current speed exceeds 120 km / h railway rolling stock were installed electronic anti-skid to prevent sliding axle. In principle, the principle of the fast truck electronic anti-skid device and the passenger car electronic anti-skid device is the same: the anti-skid device collects the four axle speeds on the vehicle in real time, and compares the vehicle reference speed to calculate the vehicle slip rate and deceleration. According to the multi-slip criterion, it is judged whether the axle is slipping, thereby adjusting the brake cylinder pressure to prevent the wheel from sliding. The principle of the electronic antiskid device designed in this paper is shown in Figure 1. 
2 hardware design
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The anti-skid device needs to collect the axle speed signals of each car. The speed sensor output signal is generally in the form of a speed pulse, and the corresponding data processing can be used to complete the anti-skid control. Higher calculation speed and data processing capability can shorten the response time of the anti-skid device and improve the response accuracy. In this paper, TI's 32-bit fixed-point DSP-TMS320F2812 is selected as the core controller of the anti-skid host. The capture unit of the event manager can capture the jump of the external pin, which can be conveniently used for the measurement of the speed sensor signal. The chip has a data processing capacity of 150 MIPS and integrates a wealth of on-chip peripherals. It has digital signal processing capabilities, powerful event management capabilities and embedded control capabilities. It is especially suitable for measurement and control domains that require large-scale data processing. [3]. In addition, the main modules used to complete the anti-skid control include: event manager (EV) timer unit and CPU timer, peripheral interrupt extension module (PIE), analog-to-digital conversion module, SPI communication interface, watchdog, General purpose I/O, external interrupt interface, and memory interface.
In order to reduce signal interference between the system input and output and the core control unit and to facilitate maintenance, the anti-skid hardware adopts a modular design as shown in FIG. 1 and is divided into a signal conditioning module, a core control unit and a driving module.
The core contains a minimal system control unit, comprising of TMS320F2812, clock and reset circuit, power supply and filter, the JTAG like. Further, the electronic anti-skid device also comprises other necessary application circuit, such as power-down fault code memory, display, and clearing. In this paper, the serial peripheral interface (SPI) of DSP is extended to EEPROM storage. X5045 is selected as the fault code memory, and its storage capacity is 4 KB, which can be erased by 1 million times. In order to facilitate maintenance, this paper designed the fault code display function, using MAX7219 as the display driver of two digital tubes, controlled by the I/O port of DSP. In addition, the three external interrupt ports of the TMS320F2812 are used directly as a button control interface for controlling the display and clearing of fault codes.
The signal conditioning module and the drive module are important components of the electronic anti-skid device. The signal of the speed sensor is connected to the 4-channel capture port of the DSP through signal conditioning (optocoupler isolation). The speed of each axle is collected by the DSP in real time, and the slip ratio and deceleration are calculated. The anti-skid device controls the solenoid valve drive circuit according to the operation condition of the axle, thereby controlling the anti-skid valve to charge and deflate and adjust the brake pressure.
3 software design
On the basis of clarifying the function of the electronic anti-skid device, the system adopts the idea of ​​modular programming. The overall block diagram of the program is shown in Figure 2. Because the TIE320F2812's PIE module can support up to 96 interrupts, it brings great convenience to the modular programming of the system and enhances the readability of the system program. Among them, speed processing and taxi detection control are the key modules to realize the system function. 
3.1 Calculation of slip rate and deceleration The axle forward speed v is proportional to the pulse frequency f from the axle sensor: 
Where D is the wheel diameter and Z is the number of pulses per wheel forwarded. As long as the speed sensor pulse frequency is measured, each axle speed can be calculated. Because the vehicle brake process speed sensor frequency varies widely, in order to ensure high measurement accuracy in high and low frequency conditions, this paper selects the variable cycle M/T method speed measurement, as shown in Figure 3. The maximum error of the frequency measurement is a reference pulse signal, where the frequency of the reference pulse (37.5 MHz) is equal to the system clock frequency divided by the division factor. Therefore, the frequency of the velocity pulse can be calculated by measuring the number of reference pulses corresponding to the velocity pulse. Capture CAP1, the time base of the 2 pin is timer 1, set the counting period to 1 ms; capture CAP5, the time base of 6 feet is timer 3, and the counting period is 1 ms. Turn on the timer period interrupt and capture interrupt, the number of reference pulses corresponding to N speed pulses: 
The accuracy of the speed measurement directly affects the control results, so the anti-skid device has a higher requirement for speed measurement. In this paper, a pulse signal is generated by a function signal generator, the speed sensor speed output is simulated, and the frequency is measured by an electronic anti-skid device to verify the speed measurement accuracy of the electronic anti-skid device. Table 1
The experimental results are given. Among them, fosc is the frequency of the signal generated by the function signal generator (DF1405), vosc is the corresponding speed for each 200 pulses of the axle, fDSP is the frequency measured by the anti-skid, and vDSP is its corresponding speed value. The results show that the absolute error of the speed measurement of the electronic anti-skid device does not exceed 0.05 km/h at high speed or low speed, and the relative error does not exceed 0.05%, which can meet the requirements of the anti-skid device for speed measurement accuracy. 
3.2 taxi detection and control
This paper applies fuzzy control algorithm to realize anti-skid control. In order to improve the efficiency of DSP operation and shorten the response time of the system, the system adopts the offline query method to realize fuzzy control. The two-dimensional fuzzy controller is designed by MATLAB/Simulink simulation. The basic domain of slip rate is [0, 0.25], minus The basic field of speed is [-4, 4]. In the actual control process, as long as the quantized value of the slip ratio and the deceleration is measured, the current control amount can be obtained by looking up the table. The control output of the electronic anti-skid device is the charge and discharge time (0-500 ms), the positive value is the inflation, the negative value is the abandonment, and the 0 is the pressure holding.
Set the CPU timer 0 cycle interrupt to 5 ms (that is, the single charge and discharge bleed time is 5 ms), and set the CPU timer 2 interrupt period to 100 ms [4] (ie, the coasting state detection cycle). In the actual control, the fuzzy control amount OP performs the holding pressure within [-50, 50].
4 anti-skid simulation test
In order to verify the control effect of the anti-skid device, the anti-skid simulation test was carried out in the laboratory, as shown in Figure 4. The experiment uses LabVIEW software as a platform to simulate the vehicle speed signal and the speed signal of the sliding axle. The control of the pressure of the volume chamber is controlled by the anti-skid device to control the pressure of the brake cylinder. The two-way axle signals obtained by MATLAB/Simulink simulation are read by LabVIEW on the computer, and the NI 6008 data acquisition card is controlled to generate two voltage signals proportional to the speed. The voltage signal is subjected to a voltage frequency conversion circuit to obtain a pulse signal proportional to the frequency and the speed, and the speed pulse signal is isolated from the CAP pin of the DSP via the optocoupler of the signal conditioning module. The voltage frequency conversion is implemented by using the AD654 chip. The AD654 is a low drift, high linearity, low cost voltage to frequency conversion chip that requires very few external components for voltage conversion. 
The test object is the volume chamber, the pressure is controlled by the inflation valve and the exhaust valve, the pressure of the volume chamber is connected to the data acquisition card through the pressure sensor, and the pressure of the volume chamber is displayed in real time through LabVIEW programming on the computer.
The test simulates a situation in which the axle is slipped twice during the braking process, and the result is shown in FIG. 5. When the anti-skid device detects that the axle is sliding, the pressure of the volume chamber is adjusted in real time, which prevents the axle from continuing to slide, and continues to restore the pressure of the volume chamber after no taxiing, so as to ensure the smooth implementation of the braking force. 
Anti-skid control is one of the key technologies for fast truck braking, which is of great significance to ensure braking safety and improve braking efficiency. The DSP-based fast truck electronic anti-skid device designed in this paper can measure the axle speed with high precision, and use fuzzy control as the algorithm to detect the axle sliding state according to the multi-slip criterion, adjust the brake cylinder pressure in time, and prevent the axle from continuing to slip, thus ensuring Safe braking, shorten braking distance and improve braking efficiency. The simulation test shows that the anti-skid device has the characteristics of fast response, good real-time performance and high accuracy.
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