Development of BCU single board test and diagnosis test bench based on LabVIEW

Application areas: EMU brake control unit, circuit board detection, fault diagnosis, signal acquisition

Challenge: The Brake Control Unit (BCU) is the core component of the EMU brake control system, responsible for braking force calculation, electric-pneumatic combined brake algorithm control, anti-skid control, air compressor control, communication, monitoring and fault handling. At present, the detection and diagnosis of BCU focuses on the overall function detection of BCU equipment. Once there is a fault in the equipment, the fault circuit cannot be located, which may eventually cause damage to the entire equipment, increase maintenance costs and even endanger the reliable, safe and stable operation of the EMU. Therefore, the development of a highly integrated, automated and intelligent single-board fault testing and diagnosis system for factory inspection and regular maintenance of large-scale products has important practical significance and economic value.

Application plan: This test bench is designed to implement single-board testing of the EMU brake control unit, and can realize functional testing of the power board, interface board, anti-skid plate, CPU board, and communication board. This test system adopts a technical solution that combines a top-down layer-by-layer design method with a bottom-up layer-by-layer implementation method. First, on the premise of clarifying the characteristics of the object to be tested and the test tasks, the hierarchical model of the overall structure of the system is analyzed from top to bottom, and the overall design framework of the system is established. According to the different design purposes and functions, the sub-module design and software and hardware division are carried out, and the overall design of the software and hardware of the test system is carried out. Secondly, in the process of implementing the test system, on the basis of building the basic application framework of the test system, the definition of the basic data structure and the interface definition between the modules are completed, and then starting from the basic module, the design of the key modules is completed, and then the design of other modules is gradually completed, and finally the design of the entire test system is completed.

Products used:

LabVIEW 2009 Software Development Platform

PXI-1045 Chassis

PXI-8108 Embedded Controller

PXI-4070 Digital Multimeter

PXI-2503 Matrix Relay Switch

PXI-2566 High Current Relay Switch

PXI-2570 Multi-Channel Universal Relay Switch

PXI-6528 Digital Input and Output Module

PXI-6723 Analog Output Module

PXI-6220 Analog Input Module

PXI-6602 Counter/Timing Module

PXI-2575 Switch Module

And matching junction box and cables

text:

I. Introduction

With the rapid development of high-speed railways in recent years, the safety of high-speed train operation has attracted people's attention, so the reliability of high-speed train braking systems has become more important. The brake control unit is the core component of the high-speed EMU brake control system, responsible for braking force calculation, electric-pneumatic combined brake calculation control, anti-skid control, air compressor control, communication, monitoring and fault handling. At present, the detection of BCU focuses on the overall function detection of BCU equipment. Once the equipment fails, the fault circuit cannot be located, which may eventually lead to damage to the entire equipment and increase maintenance costs. Therefore, it is of great practical significance to develop a highly integrated, automated and intelligent single-board fault testing system for factory inspection and maintenance of mass-produced products, and to use circuit board fault diagnosis technology to perform single-board testing and fault circuit location on the BCU brake control unit.

In view of the large-capacity data collection work of BCU and the high-reliability test requirements of railways, the use of traditional discrete measurement and data processing equipment will be restricted by data transmission rate, equipment footprint, test environment and other conditions. This test bench adopts NI's PXI bus-based data acquisition system, which uses PXI embedded controllers to cooperate with data acquisition boards, and flexibly configures the functions of each data acquisition board through software control, maximizes the reuse of hardware, greatly simplifies the system structure, and improves the data transmission speed. The entire test system has higher data throughput and test rate than the whole machine test system.

2. Design background and development concept of BCU single board detection system

The brake control unit includes five circuit boards: CPU board, interface board, anti-skid board, power board and communication board. Figure 1 is a structural diagram of the working principle of BCU. Through the data and command information transmitted by the driver controller, train sensors and train communication network, BCU performs complex braking and anti-skid calculations, outputs the braking force currently required by the train, drives the relay valve and anti-skid valve, and controls the brake air cylinder to complete the train braking.

Figure 1 Schematic diagram of the working principle of the brake control unit

The single-board test bench inputs information from the human-computer interaction interface, simulates the data transmitted by sensors and train networks, simulates the braking process, and compares it with the data detected by the data acquisition system after calculation to verify the working status of each BCU board. It mainly includes three tasks: first, according to the control instructions of the main controller PXI-810, the data output board outputs analog, digital, PWM and other signals to each BCU circuit board, providing the original excitation model for the circuit board; second, perform simulation operations to calculate the output values ​​of each signal of the BCU board under the current conditions; finally, the BCU board output is collected with high precision to complete the comparison, identification and storage of data.

The system design mainly follows the following concepts:

1. Give full play to the advantages of virtual instrument technology, use software configuration instead of hardware resources, and maximize the utilization of data acquisition board hardware resources;

2. Overall planning of the single-board test bench hardware and software resource configuration, dividing the test content into 17 test items in 10 categories, including conduction test, characteristic test, voltage measurement, analog output loop test, PWM frequency/pulse width test, etc. The same test items of different circuit boards can be combined into one group for testing, so as to reuse hardware resources and speed up the test;

3. The test realizes automatic testing and manual testing, and opens the underlying hardware control rights to users to the greatest extent possible to facilitate secondary development;

4. Use the high-performance processor motherboard based on Intel Core Duo T9400 provided by NI to achieve multi-threaded operation and speed up the test rate;

5. Intelligent testing, the system supports auxiliary functions such as test data saving and printing, automatic generation of test reports, etc.;

6. Use touch screen as the human-machine interface, with rich and friendly interface.

3. Overall design of BCU single board test system

When building a BCU automatic test system, the versatility and scalability of the hardware platform must be considered. The entire platform can adapt to changes in the state of the device under test, and at the same time has strong expansion and upgrade capabilities based on existing hardware resources, and meets the system's miniaturization requirements. This system uses an embedded control method and a 3U PXI module. The data acquisition unit and signal simulation unit are based on the PXI bus and are implemented in the PXI chassis, which is the core of the entire automatic test system.

This system uses embedded control and 3U PXI data acquisition modules based on PXI bus. The data acquisition unit and signal simulation unit are implemented in the PXI chassis. The PXI data acquisition boards and functions used in the system are shown in the following table (except relays and controllers).

3.1 Overall hardware design

This system uses PXI bus and PXI embedded industrial computer. In terms of hardware resources, it uses powerful modular virtual instruments based on PXI bus, giving full play to the advantages of PXI bus, thus providing hardware guarantee for accurate and fast testing of circuit boards. The test system is mainly composed of the following parts: the main control unit mainly processes and analyzes the signal, the signal output part provides excitation signal to the board under test, the signal acquisition part collects the output signal of the board under test, the signal conditioning board conditions the signal and converts the interface, and the power supply part supplies power to the board under test, digital board and adapter board. The overall design structure of the system is shown in Figure 2. [page]

Table 2 is a statistics of the minimum number of channels required for system common resources.

This system mainly includes power supply module, five boards under test, a transfer board, a PXI bus chassis, a touch LCD display and a printer, etc. The boards under test are connected to various acquisition, output and measurement boards in the PXI bus chassis through the conditioning board. The upper software is programmed with LABVIEW to realize the automatic test of the system. The touch LCD display is the human-computer interaction interface, and the printer can print out the test results.

Figure 2 BCU single board test system structure diagram

3.2 Overall design of the software

The parameter setting, instrument configuration, test stimulus generation, response data collection, test result judgment, fault location and report generation of the test system are all completed under the control of the system software. This test system refines the detection of the EMU brake control unit to the circuit board level. The software design of this test system follows the modular design principle, uses the LabVIEW development environment for programming, and uses software programming to configure and use hardware resources to the greatest extent, which greatly improves the system's versatility, reusability and scalability. A rich human-computer interaction interface is designed to ensure that the test operation is intuitive and simple, and can display test data in real time and locate fault circuits.

The test software is mainly composed of three modules: circuit board automatic test module, test information maintenance module, and human-computer interaction module. It is designed to include test process management and control software, test subroutines, and data management subroutines. It can also automatically generate reports for easy query and printing. The schematic diagram of the main functional modules of the software is shown in Figure 3.

In order to improve the compatibility and scalability of the system, this experiment has designed a detailed data maintenance system for the data flow during the test process. Users can read, store, and analyze test data through the main control computer according to test requirements, and generate test reports and print them; users can also log in to the system, add or delete relevant test information, and maintain and modify test data.

Figure 3 Software Function Flow

4. Test bench development and achievement display

The following discusses in detail the BCU single board test process developed using NI's PXI data acquisition system and LabVIEW graphical development platform from the two aspects of analog quantity measurement and PWM quantity measurement.

4.1 Analog Measurement

Analog measurement includes two hardware drivers: analog output and analog acquisition, as well as three important links: software filtering. Analog output includes 0-10V sine wave and square wave, 0-10V DC voltage and 10-30mA current. The analog output is completed using NI's PXI-6723 module and the corresponding V/I conversion module. Analog tests include 0-20V voltage, 10-30mA current, and 0-700mA current. The PXI-6220 board is used, and the analog acquisition is completed with the corresponding sensors and conditioning circuits. The analog output and acquisition function block diagram is shown in Figure 4:

Figure 4: Analog output and acquisition function block diagram

When measuring the EP current output of the CPU board, the excitation signal needs to simulate the train digital input, speed input signal and regenerative feedback voltage signal. And the above input signals must be provided at the same time, so that the EP valve of the board under test can generate the EP valve output current. Therefore, it is necessary to perform output detection operations in parallel to detect the output of EP current during the change of shaft speed. Using the hardware driver and related function library provided in LabVIEW, it is possible to achieve a simple parallel output of the excitation signal and the acquisition of EP current.

Figure 5 Analog output and acquisition program

Since the current sensor has a long transmission distance, the line will introduce some noise. Using the signal analysis and processing toolkit in LabVIEW, the filter mode is set to a low-pass filter with a cutoff frequency of 50Hz to prevent interference from power frequency and high-frequency noise. This can effectively suppress noise and improve the EP current measurement accuracy.

4.2 PWM Measurement

The detection process of PWM signal is to input pulse to the control relay first, let the relay act first, then input control signal to anti-skid control, and detect its output and feedback signal at the same time. The detection of PWM signal includes frequency and duty cycle, because the PXI-6602 acquisition board is used in the test process.

Figure 6 PWM acquisition function block diagram

The task completion process of frequency acquisition is as follows:

(1) Start the pulse output task: Generate control pulses according to the channel specified by the user;

(2) Output anti-skid control pulse task: Generate anti-skid control pulse according to the user-specified channel;

(3) Pulse detection task: Create a task for collecting pulse frequency, display the collected results on the front panel, and end the task;

(4) End the pulse output task: end the pulse output and release the hardware resources;

(5) Error handling: If there is an error or warning in the measurement result, a dialog box will pop up to prompt.

The program execution diagram is shown in Figure 7.

Figure 7 Anti-skid plate PWM detection procedure

4.3 Human-computer interaction interface development

The system software interface is shown in Figure 8:

Figure 8 Main interface of the single board detection system [page]

Its operating functions include:

(1) Test implementation: Enter the circuit board test interface and perform single board tests on five types of circuit boards;

(2) Single test: Enter the circuit board hardware confirmation interface, adjust each test parameter of the five types of circuit boards, and view the test results. Mainly used for testing individual items of the test circuit board;

(3) Test result call-up: view the saved historical data;

(4) Manage master data: manage and view menu registration items, user information, test parameters, etc.; provide access to view historical data;

(5) Exit: Exit the brake control unit single board detection program.

When entering the test implementation, the system will hide the main interface and enter the single board test registration interface. The test implementation interface consists of test information registration, test implementation and other auxiliary windows. It can perform single board tests of four types of circuit boards, as well as view, save and print test data. It is a necessary step for automatic testing during test registration.

Figure 9 Automatic test interface

The manual test of the brake control unit provides a single input and output test for each test signal, which is convenient for users to detect and debug a circuit fault. It opens up the underlying hardware control rights with greater authority for users.

Figure 10 CPU board manual test front panel

The system includes a data management system based on SQL Server for historical data and test process data, and a management system for data viewing, modification and printing based on multi-level user permissions.

Users need to authenticate their identity when performing advanced data operations. Historical data query provides data query operations for all previous test items.

Figure 11 Identity verification and historical data query

4.4 Experimental Development Architecture

In summary, for the circuit board under test, due to the large number of paths to be tested and the complex functions, the task of completing the test software is arduous. Therefore, this system adopts a hierarchical modular design for human-computer interaction, experimental testing, file operations, etc., which is gradually decomposed from the upper layer to the lower layer, and gradually executed from the bottom layer to the upper layer, and the corresponding sub-VI is generated for the upper layer to call. The lower layer is provided with parameter configuration by the upper layer, and the result data is returned to the upper layer for processing. The mutual call in LabVIEW is used to the greatest extent to realize the reusability of hardware and the modularization of software.

Figure 12 Subroutine structure diagram

4.5 Field testing

Figure 13 Field test diagram

Taking the CPU board EP current test and anti-skid plate PWM output test in the characteristic test as examples, the single board test process and analysis results of the brake control unit are explained.

The EP current test results of the trailer CPU circuit board under empty conditions are shown in Figure 14. The CPU boards that work normally and those that fail are analyzed and compared. According to the design requirements of the brake control unit, the allowable error of the EP current output is ±15mA. Figure 14 (a) shows the EP current output of the brake control unit under rapid braking of the CPU board without fault; Figure 14 (b) shows the EP current output of the CPU board with fault under rapid braking; then the manual single-channel test determined that the AS1 and AS2 pressure input acquisition circuits of the CPU board failed, resulting in low output EP current. The test results prove the reliability and accuracy of the test system.

Figure 14 EP current test results comparison

In the test of whether the 100V PWM output characteristic function of the anti-skid plate is normal, the PXI 6602 board is used to configure the PWM with an output amplitude of 5V and a duty cycle of 23% in LabVIEW. The 110V PWM output waveform of the anti-skid plate is shown in Figure 15. It can be seen that the performance of the anti-skid plate is normal, which also verifies that the output accuracy of the PXI 6602 board is high and fully meets the test requirements.

Figure 15 Field test waveform

2. Conclusion

Based on the analysis of the resources required for the BCU single-board test, the PXI system was used as the hardware test platform. Through the analysis of the technical requirements of the circuit board test system and the study of the composition principle of the test system, the hardware design of the test system and the design of the system signal conditioning board were completed. In terms of software design, LabVIEW software was used to follow the modular design method to complete the design of the automatic test subroutine and test interface, the design of the human-computer interaction visualization interface, including the system startup initialization interface, the test implementation interface, the manual test interface, and the management of the main data interface. By operating the human-computer interaction interface, the tester can call the test subroutine and finally complete the test of the circuit board. At the same time, the test results can be saved and printed. The actual test results verified the feasibility and reliability of this system and achieved the expected design goals. The software and hardware products provided by NI provide efficient, convenient and flexible support for the development of our EMU brake control unit single-board test bench, shorten the development cycle, and are unanimously recognized by users.