Design and application of high-speed solenoid valve in adaptive single-vehicle test system

The train braking process is completed by exhausting and reducing pressure in the tubes at the bottom of the vehicle to inflate and pressurize the brake cylinder to drive the brake. The single-vehicle test system is a tool that simulates various standard exhaust processes of the train tubes and then tests the brake.

With the commissioning of 70 t freight cars, the length of train cars increased from the initial 14.5 m to the longest 26.5 m, and the volume of the tubes at the bottom of the cars also increased accordingly. However, when using the single car test system to test the train brake, the tube decompression and exhaust curve is required to be consistent with the original standard tube exhaust curve of 15.5 L to meet the use conditions of the brake. The original single car test system can only simulate the standard decompression and exhaust process when testing 45 t cars with a length of 14.5 m and a tube system volume of 15.5 L. Therefore, the original single car test system can no longer meet the test requirements of the Ministry of Railways for train brake systems.

To this end, the original single-vehicle test system was transformed and upgraded, and the pulse width modulation high-speed solenoid valve (PWM high-speed solenoid valve) was used to automatically adjust the exhaust aperture to compensate for the original exhaust process, so that the decompression exhaust process of different volume tubes can meet the process requirements. The adaptive single-vehicle test system has been successfully applied to the production inspection of Taiyuan Vehicle Factory and Luoyang Vehicle Depot.

1. Formulation of the plan

It is required that any volume of tubes can automatically achieve the exhaust decompression process required by the Ministry of Railways for brake machine testing during the exhaust process of the test, which means changing the corresponding decompression exhaust aperture, that is, changing the exhaust aperture of the corresponding exhaust solenoid valve on the basis of the original single-vehicle test system.

After experiencing the manual single-vehicle test system, micro-controlled single-vehicle test system, and DCS micro-controlled single-vehicle centralized test system, in recent years, some people have also used incremental digital flow valves in single-vehicle test systems to adapt to various models with different tube volumes. The principle is to use a stepper motor to control the threaded adjustment rod to convert the angular displacement into axial displacement, so that the adjustment spring in the valve obtains compression, thereby changing the exhaust aperture of the exhaust solenoid valve, so that the exhaust pressure reduction curve of the tube is the same as the standard curve. However, in practical applications, this method has the disadvantages of high cost, complex control objectives and algorithms, so this single-vehicle test system cannot achieve the expected control effect in practical applications. The adaptive single-vehicle test system uses PWM high-speed solenoid valves as the main means of flow rate regulation, and adaptively adjusts the exhaust process during single-vehicle testing, so that the exhaust pressure reduction curves of trains with different tube volumes can meet the process requirements of the Ministry of Railways. The PWM

high-speed solenoid valve controls the high-speed switch of the valve port by a modulated pulse signal to change the time ratio (duty cycle) of the valve port opening and closing, thereby adjusting the average flow or pressure output by the valve. It has strong anti-interference ability and high control accuracy, and has been widely used in pneumatic systems controlled by computers in real time.

The adaptive single-vehicle test system uses a PWM-controlled high-speed solenoid valve as a compensation valve. During the single-vehicle test, the system terminal state is adjusted in real time, so that the exhaust curve of the tubes during the single-vehicle test meets the standard exhaust curve formulated by the Ministry of Railways. This control method can adapt to the exhaust requirements of tubes with different volumes. In a short time, with the fastest response speed, the exhaust curve of the test object can track the ideal exhaust curve, thereby meeting the test requirements. This system uses VB as the programming language of the host computer and Siemens 200 series PLC as the lower computer. The classic PID control principle is used to adjust the pulse duty cycle of the PWM high-speed solenoid valve in real time, so that all indicators in the test process meet the process requirements.

2 Analysis of PWM high-speed solenoid valve characteristics

In the process of using PWM technology to control gas, due to the slow time-varying characteristics of the gas, the high-speed solenoid valve acts as an integrator, so the flow rate can be controlled by controlling the flow rate change rate of the valve. The equivalent area Sc of the solenoid valve during its operation can be expressed as:

Where: SPWM is the output of the high-speed solenoid valve. When the frequency of the PWM signal is high enough, due to the low-pass filtering characteristics of the system loop and the system components themselves, the gas dynamic signal is a continuous slow-changing signal carrying certain frequency signals. In this way, the high-speed switch valve also exhibits the function of digital/analog conversion under the action of the signal. This shows that the introduction of PWM control mode on the traditional switch solenoid valve can achieve continuous control of the fluid dynamic signal.

When a PWM pulse with a voltage amplitude of U and a time width of T is applied to the high-speed solenoid valve, the average flow rate of gas passing through the high-speed switch valve in each cycle is Q, then:

Where: Qmax is the maximum flow rate through the valve when the valve port is fully open (unit: L/min); D is the duty cycle; Cd is the flow coefficient of the high-speed switch valve; Av is the valve port flow area (unit: cm2); △P is the valve port pressure difference (unit: MPa); ρ is the oil density (unit: kg/cm2).

Formula (2) is an ideal formula. Since the valve core displacement waveform is distorted, the actual duty cycle has changed, so formula (2) should be corrected. This correction amount increases with the difference of the above 6 parameters. The actual average flow formula should be:



Where: k is the correction coefficient. It can be seen from formula (4) that by adjusting the duty cycle D, the average flow through the switch valve can be continuously controlled, and the accurate and continuous control of the flow can be achieved, and finally the control of the output port pressure can be achieved.

3 System Design

3.1 System Gas Path Structure Diagram

Before using PWM to control the high-speed solenoid valve, an exhaust solenoid valve with a corresponding aperture should be added to the exhaust port of the working chamber according to the different volumes of the tubes. With the continuous increase in railway speed and train load, the volume of the tubes at the bottom of the train is constantly changing, so the number of corresponding solenoid valves must also be continuously increased. However, as shown in Figure 1, in the adaptive single-vehicle test system using PWM high-speed solenoid valves as compensation valves, the solenoid valves of various exhaust apertures originally connected to the working chamber are replaced by a solenoid valve with a fixed aperture and a high-speed solenoid valve, and the connection points are greatly reduced. This not only increases the scope of application of the test system, but also reduces the number of terminal accessories and reduces the difficulty of production and maintenance.

3.2 Design of the system control part

The host computer program of the adaptive bicycle test system is written in VB. The entire host computer program is divided into three parts: communication part, data recording management part and control display part. The communication part uses the MSComm control of VB to complete the two-way free port communication with the lower computer PLC, and sends the operation instructions of the host computer to the lower computer in real time, and controls the output results of the PLC by changing the corresponding register content in the PLC. At the same time, the pressure data read by the lower computer is transmitted to VB and accepted by the MSComm control for further processing. Since a large amount of data recorded by the system test must be saved in the database, the data recording management part uses ADO technology to establish a connection with the Access database to complete data access. The Access database file and the VB program exist independently of each other, which ensures that the experimental data can be stored for a long time. As the human-computer interaction interface of the system, the control display part converts the operator's operation instructions into control signals and provides them to the communication module to send to the lower computer, and processes the pressure data received by the communication module and converts them into an intuitive curve of pressure change over time for recording.

Since the high-frequency pulse output function is required, the DC/DC226PLC in the Siemens S7-200 series PLC is used as the lower computer of the system. The PLC program adopts a structured method and consists of three parts: initialization program, data acquisition program and exhaust test program, as shown in Figure 2.


The initialization program mainly includes three aspects: initialization of PWM function, initialization of free port communication and initialization of PID function. When testing the standard 15.5 L volume tube, the data acquisition program uses a 20 ms timing interrupt to collect pressure values ​​in real time and store them in the specified register. After the experiment, it is uploaded to the upper computer through the communication program as a standard value storage record.

The exhaust test program is the key part of the whole system. It includes data download, PID closed-loop control and PWM signal output. Before exhaust, the collected discretized standard curve is downloaded from the host computer to the register specified by the PLC, and a pointer variable is used to point to the first data. When exhaust starts, the first data is passed to the PID module as the standard quantity. The PID module calculates and outputs the corresponding result according to the current pressure value collected by the analog input module 231 and the standard quantity. This result is converted into a duty cycle and then output by Q0.0 of the PLC. The modulated pulse is output. Since the output of the PLC cannot directly drive the PWM high-speed solenoid valve, a solid-state relay should be added between the PLC and the solenoid valve to amplify the pulse signal output by the PLC. The output pulse of the PLC can finally control the opening and closing of the PWM high-speed solenoid valve. During the exhaust process, the pointer variable moves to the next data every 20 ms and passes it to the PID module; the PID module adjusts the duty cycle of the PWM signal according to the constantly changing standard quantity and the pressure value at that time during the whole experiment, so that the exhaust pressure curve of the tubes with different volumes is consistent with the standard 15.5 L exhaust pressure curve.

4 Trial operation of adaptive single-car test

According to the requirements of the "Railway Freight Car Brake Equipment Maintenance Rules" issued by the Ministry of Railways of the People's Republic of China in 2008, the adaptive single-car test system using PWM high-speed solenoid valves was subjected to functional inspection (referring to the inspection of whether the single-car tester itself is qualified, which is a railway-specific term), and the single-car tester was connected to the train pipe volume calibration air cylinder with a volume of 15.5 L. The exhaust time of each position met the maintenance regulations.

After the functional inspection was qualified, the adaptive single-car tester system using PWM high-speed solenoid valves was further tested to simulate various braking processes under the conditions of a 70 t-class freight car with a tube volume of 17.5 L. First, the tester simulated the exhaust pressure process of the stable test position (for 120 valves and 120-1 valves). The stable test position test requires recording the change process of the tube pressure from 500 kPa to 300 kPa. The standard tube with a volume of 15.5 L and the tube with a volume of 17.5 L were tested respectively. The test of the tube with a volume of 17.5 L was divided into two cases: opening the PWM high-speed solenoid valve and not opening it. The pressure change over time during the above test was recorded and a curve was drawn, as shown in Figure 3. The dark curve in the figure is the pressure change curve of 17.5 L, and the light curve is the pressure change curve of 15.5 L. The jitter of the dark curve following the light curve is because the pressure measuring point is too close to the high-speed solenoid valve, which does not affect the operation of the brake. As shown in Figure 3, when the 17.5 L tube was tested without opening the PWM high-speed solenoid valve, the pressure change curve obviously deviated from the standard change curve of 15.5 L; the change curve measured after opening the PWM high-speed solenoid valve for the test closely followed the standard curve. This result shows that when the volume of the measured object changes significantly, the use of PID to control the PWM high-speed solenoid valve to adjust the exhaust aperture can ensure that the pressure change curve over time is still basically consistent with the standard 15.5 L pressure change curve, meeting the expected requirements.



In addition, the 5-position stable test position (tube pressure drops from 500 kPa to 300 kPa within 6.5 to 9 s), the emergency brake position (tube pressure drops from 500 kPa to 200 kPa within 3.5 to 5 s) and the 120 valve and 120-1 valve emergency brake position (tube pressure drops from 500 kPa to 200 kPa within 1.5 to 2.5 s) were tested under the 15.5 L standard condition and the 17.5 L Kaitang PWM high-speed solenoid valve condition. When the 17.5 L PWM high-speed solenoid valve is turned on, the exhaust is performed slowly while the air is filled to simulate the interference condition. The above tests record the pressure data and draw the curves, as shown in Figure 4.



The dark curves recorded in Figure 4 when the PWM high-speed solenoid valve is turned on at 17.5 L all follow the standard curve changes very well. These tests further show that the adaptive single-vehicle test system fully achieves the expected purpose and meets the experimental requirements.

5 Conclusion

Practice has proved that after the successful development of the adaptive single-vehicle test system, it has well solved the single-vehicle test problem of 70 t-class truck vehicles, and reduced the complexity of the pipeline part, reduced the pipeline connection, and is more conducive to ensuring the sealing of the equipment, reducing the difficulty of equipment manufacturing and maintenance. At the same time, the system can also conveniently test other special vehicles, and the scope of application of the system has also been expanded.