Application areas: hydraulic system control, signal acquisition, test condition monitoring, data processing
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Challenge: The connecting rod is one of the key components of the engine, and its reliability plays a vital role in the life of the machine. In order to study the fatigue characteristics of the engine connecting rod under tension and compression load, the developed engine connecting rod test bench is based on NI LabVIEW development environment and CompactRIO embedded controller, and uses hydraulic servo loading method to pull and load the connecting rod. . How to determine the loading load and loading frequency according to the parameters of the engine and connecting rod? How to control the hydraulic system to load according to the set loading load and frequency? How to monitor the test state in real time, judge the fatigue damage of the connecting rod? How to collect, store and analyze the relevant The data yields the corresponding test results? This is the key to the entire test system.
Application: CompactRIO includes a real-time controller and reconfigurable field-programmable gate array (FPGA) chip and includes 8 hot-swappable industrial I/O slots for excellent reliability and real-time performance. The input and output boards for various types of signals are matched with CompactRIO to form a complete hardware system. NI LabVIEW includes powerful functions of control, acquisition, monitoring, analysis, etc., providing a complete software design platform for the entire test system.
Products used:
LabVIEW 2009
LabVIEW RT 9.0
LabVIEW FPGA 9.0
LabVIEW PID Control Toolkit 9.0.0
CompactRIO-9014 reconfigurable embedded chassis
CompactRIO-9014 embedded controller
CompactRIO-9237 strain signal acquisition module
CompactRIO-9205 voltage signal acquisition module
CompactRIO-9263 voltage signal output module
CompactRIO-9401 DIO Module
CompactRIO-9485 SSR Module
text:
I. Introduction
Strength, stiffness and fatigue life are the main parameters for measuring the reliability of engineering machinery and parts. Fatigue damage is one of the main causes of failure of mechanical mechanisms and parts. According to statistics, 60% to 90% of the damage of connecting rods is fatigue damage. The main cause of fatigue damage is dynamic alternating load.
The connecting rod is the core component of the reciprocating piston internal combustion engine and one of the main components of the internal combustion engine that bears a large alternating load. Its reliability directly affects the safety of the internal combustion engine.
At present, the main means of research on link fatigue reliability include simulation calculation, real machine test and simulated fatigue test.
The simulation calculation is convenient, fast and low cost. There are many simulation calculations for the connecting rod, but only the fatigue reliability can be analyzed and verified, and the boundary conditions are uncertain. The real machine test can reflect the real working condition of the connecting rod. However, the test period is long, the cost is high, and the intensification test cannot be performed; the simulated fatigue test can be tested with a shorter period of higher efficiency, and the fatigue test can be strengthened to more comprehensively test the fatigue characteristics of the connecting rod.
At present, most of the domestic equipment is imported from abroad, and there is no self-developed connecting rod tension and compression simulation test bench. Therefore, it is of great significance to develop a simulated fatigue test bench for the engine connecting rod. In the development of the test bed, NI LabVIEW development environment and CompactRIO embedded controller and its supporting boards provide us with powerful tools.
II. General introduction of the test system
2.1 Connecting rod stress
As shown in Fig. 1, during the running of the engine, the movement state of the connecting rod is relatively complicated, the small head makes a reciprocating motion, the big head makes a rotary motion, and the shaft body performs a plane motion. At the same time, the force of the connecting rod is also very complicated. The force of the connecting rod in the actual working condition can be divided into two parts: one part is the gas burst pressure generated in the work and the reciprocating inertial force of the piston assembly; the other part is the connecting rod The inertial forces generated during the movement, including the reciprocating inertial force, the oscillating centrifugal force and the transverse bending moment (the transverse bending moment is relatively small, and its extremum does not appear with other forces, so it is ignored).
Under the action of the above forces, there will be bending moment, shear force and normal force on each section. However, the bending moment and the shearing force are not much compared with the normal force, and the connecting rod mainly bears the alternating tensile and compressive loads.
2.2 Test principle
The design of the test bench mainly considers the tensile compression load of the connecting rod in actual operation, ignores the bending moment and the shearing force, and pulls and compresses the connecting rod. Although this design can not completely simulate the actual working condition of the connecting rod, it can basically reflect the tensile and compressive fatigue characteristics of the connecting rod accurately, and achieve the purpose of the bench simulation test.
The test bench can perform static load test and dynamic fatigue test on the connecting rod test piece. In the static load test, only the tensile force or pressure is slowly applied to the connecting rod test piece to investigate the static material characteristics of the connecting rod; in the dynamic fatigue test, the alternating tensile and compressive load is applied to the connecting rod test piece, due to the actual operating conditions of the engine. The maximum compressive load of the rod is greater than the maximum tensile load, and the test bench adopts an asymmetric loading mode, that is, the load ratio is not -1. After the test is completed, the test system will save all test data, including strain signal, tension and pressure load signal, cycle number, fatigue damage state and so on. Through the statistical analysis of the test data, the fatigue life evaluation of the connecting rod and its reliability design are realized.
The control of the whole test system is realized by NI LabVIEW development environment and CompactRIO embedded controller, including hydraulic load control, data acquisition, storage, analysis, test system monitoring and safety control.
I. Overall design of the test system
The design of the entire test system can be divided into two parts: hardware and software.
3.1 Hardware Design
The hardware of this test bench mainly includes mechanical platform, hydraulic loading system and control system.
3.1.1 Mechanical platform
As shown in Figure 2, the mechanical table body adopts a four-column structure. The main function is to fix the connecting rod test piece and support other mechanisms for testing. It mainly includes the connecting fixture fixing fixture, the moving rail, the moving surface and the supporting surface.
3.1.2 Hydraulic loading system
The main function of the hydraulic loading system is to provide a pre-set loading load for the simulated fatigue test. Its structure is shown in Figure 3. It mainly includes hydraulic servo solenoid valve, hydraulic amplifier, hydraulic cylinder, hydraulic pump station and accumulator. It also includes auxiliary hydraulic devices such as oil filter, check valve and relief valve.
The hydraulic servo solenoid valve adopts the three-position four-way valve of Rexroth, Germany, in which P is the high pressure oil circuit, T is the return oil circuit, A and B are the working oil lines; the hydraulic amplifier is used to amplify the hydraulic pressure to provide more Large loading load; the hydraulic cylinder is a single-rod double-acting piston hydraulic cylinder for carrying out the loading of the connecting rod test piece; the accumulator is used to keep the pressure of the test system stable, and the two accumulators are respectively located in the return oil passage and the high pressure Oil circuit; oil filter is used to filter hydraulic oil; check valve is used to prevent pressure oil from flowing back to the hydraulic pump station; overflow valve is used to prevent excessive pressure and keep the system pressure below the specified value.
The hydraulic servo valve realizes the tensile loading and compression loading of the connecting rod by changing the on and off states of P, T and A, B to move the hydraulic cylinder up and down.
3.1.3 Control System
The control system is an NI hardware combined with the LabVIEW development environment, including the embedded controller CompactRIO 9014, NI 9237AD module, NI 9205AD module, NI 9263DA module, NI 9401DI/O module, and NI 9485SSR module.
As shown in Figure 4, the NI CompactRIO embedded measurement and control system and the PC form the upper and lower structure of the entire test system. Among them, CompactRIO 9014 has good reliability and real-time performance, and can easily realize the measurement of corresponding variable, tension and load signals and the control output of servo solenoid valve. The working conditions of each module are as follows:
1) The NI 9237AD module is used to collect the strain signal and the connecting rod tension and load signal. The strain gauge is attached to the position of the connecting rod specimen. The strain signal is directly connected to the NI 9237AD module through the strain gauge. The tensile and compressive load is measured by the Interface tension and pressure sensor. Access the NI 9237 module.
2) The NI 9205AD module is used to collect the pressure signals of the upper and lower cylinders of the piston cylinder. The upper and lower cylinder pressure signals are collected by the KISTLER pressure sensor.
3) The NI 9263DA module outputs a set pressure signal to control the hydraulic servo valve to achieve asymmetric loading of the system.
4) NI 9401DI/O module monitoring test system status information, including hydraulic system power switch signal, control cabinet power switch signal, hydraulic pump station oil pressure, temperature, leakage, liquid level signal, the corresponding signal after the failure of the indicator light alarm.
5) The NI 9485SSR module is used to cut off the corresponding part of the switch after the fault is found to ensure the safety of the test.
3.2 Software Design
The software program of this test system is based on the development environment of LabVIEW graphical programming language, and is completed with the development of CompactRIO embedded controller. Mainly realizes the control of hydraulic loading, monitoring of test status, and processing of data. The functional structure of the test system software is shown in Figure 5.
3.2.1 Control of hydraulic loading
The loading control of the test system mainly realizes three contents: automatic calculation of link load, realization of asymmetric sine wave loading, PID control of loading load.
3.2.1.1 Calculation of loading load
The calculation of the load is mainly to determine the maximum pressure and maximum tensile force of the connecting rod to determine the positive and negative amplitude of the asymmetric sine wave. Based on the analysis of the stress on the connecting rod in 2.1 and the derivation calculation of the connecting rod tensile load (here omitted), the connecting rod can be divided into three parts: the small head, the shaft and the big head to calculate the maximum tensile and compressive load. Calculation of all sections. The simplified calculation method is as follows:
(1) Under the maximum tensile load:
Small head end: 
Big head: 
Shaft: 
(2) Under the maximum compression load:
Small head end: 
Big head: 
Shaft: 
In the middle 
- Outbreak pressure, Pa
- crank angle
——link ratio = 
- crank radius of gyration, m
——Connector size head center distance, m
——Piston mass (total mass of piston, piston ring set, piston pin, piston pin snap ring), kg
——the projected area of ​​the piston top, 
—— crank angular velocity, rad/s
- Calculate the section cut to the mass of the small end, kg
- Calculate the centroid of the small end of the section to the center of the small hole, m
——Connector quality, kg
D——bore diameter, m (here is approximate, when the top diameter of the piston is known, the diameter of the piston top is more accurate)
When the engine calculation condition is selected, the maximum engine speed condition is used as the load calculation condition of the connecting rod, and the maximum burst pressure of the highest torque of the engine is substituted for the maximum burst pressure at the maximum speed, and a relatively conservative test result is obtained.
After calculating the above data, the average load, load amplitude and load ratio can be calculated.
Average load: 
Load amplitude: 
Load ratio: 
Through the comparison of the above values, especially the comparison of the average load, the load of the big end, the small end and the shaft load are selected as the loading load. For the determined loading load, the load enhancement factor can be set for the corresponding load strengthening. The strengthened load is used as the loading load for the reinforcement test.
According to the above calculation principle, the software uses the formula node in LabVIEW to achieve the above calculation. The part of the program is extracted and tested with a certain type of connecting rod as a model. The result is shown in Fig. 6.
Figure 6: Calculation of tensile load of a certain type of connecting rod test piece
3.2.1.2 Implementation of asymmetric sine wave loading
Asymmetric loading is mainly achieved by the FPGA Memory and CompactRIO 9263 modules of the CompactRIO 9014 FPGA module. FPGA Memory can record the value of the set point, and record the address of each value in order, index the address of FPGA memory to get the corresponding value; CompactRIO 9263 outputs the corresponding voltage control waveform to control the servo valve, positive and negative voltage control Stretching or compressing the load, the magnitude of the voltage controls the opening of the servo valve to change the magnitude of the tensile or compressive load.
The specific method is:
1) Make a standard sinusoidal analog waveform with a period of 1 and consist of 1024 points.
2) Convert analog waveform to constant into FPGA memory
3) Set the positive and negative values ​​of the assigned FPGA Memory to positive and negative amplification factors respectively.
4) Input the value into the CompactRIO 9263 module to generate an asymmetric sinusoidal control waveform
For example, the link calculated in the above section generates an asymmetric waveform:
Figure 7: Output Control Waveform
When it is a static load test, only the hydraulic cylinder is controlled to move in one direction, and the negative half shaft is 0 when the tension is applied; the positive half shaft is 0 when the compression is applied.
3.2.1.3 Load Load PID Control
PID is mainly for dynamic fatigue test, the test system uses the conventional PID control method. The loading load is a cyclic asymmetric tensile and compressive load. To ensure that the loading load reaches the set maximum tensile and compressive load values, the maximum tensile load and the maximum compressive load are the target values ​​of the PID control; It is subjected to the tension and compression load signal, and the maximum value is the feedback amount of the PID control. The control principle is shown in Figure 8.
Figure 8: Load load PID control schematic
Figure 9 is a partial program diagram of the FPGA PID control module in LabVIEW, which greatly simplifies the control process. Just set each interface parameter, and then connect the feedback signal to the process variable port to get the corresponding output.
Figure 9: FPGA PID module part program diagram
The load load PID control simulation is shown in Figures 10 and 11. Figure 10 shows the settings of the parameters of the PID control module (the PID parameter debugging process is omitted here). After the setting is completed, the setpoint of the PID module is changed from 0 to 14.3, and the response characteristics are obtained as shown in FIG. The time unit is 50ms per scale. In the case of a large change in Setpoint (red), Process Variable (blue) reaches Setpoint within 3s and remains stable.
The simulation results show that the load load PID control meets the requirements of the test system. At the same time, the response characteristics can be adjusted by changing the PID parameters.
Figure 10: FPGA PID Module Parameter Settings
Figure 11: Hydraulic loading system PID control response diagram
3.2.2 Monitoring of test status
The monitoring of the test state mainly includes three parts: fatigue damage, load loading and safety failure.
3.2.2.1 Monitoring of fatigue damage
The monitoring of fatigue damage is mainly based on the strain of the corresponding position of the connecting rod test piece of the CompactRIO 9237 module, and the strain waveform and the peak-to-valley value are displayed in real time on the monitoring panel. When the setting range is exceeded and the set number of times exceeds the set range, the link is judged to be broken, the test is automatically suspended, and the tester checks.
3.2.2.2 Monitoring of load loading
Loading load monitoring includes three aspects: connecting rod tension and pressure load monitoring, oil chamber pressure monitoring on hydraulic cylinder, and oil chamber pressure monitoring under hydraulic cylinder. All three loads are collected by CompactRIO 9201 module, and the waveform is displayed in real time on the monitoring interface. The measurement of the tensile load of the pull link is measured by the Interface pull pressure sensor, and the pressure of the oil chamber of the two hydraulic cylinders is measured by the KISTLER pressure sensor.
3.2.2.3 Monitoring of safety faults
It is a top priority to ensure the safety of the test. The test system monitors the part of the safety hazard and performs corresponding treatment after the fault is found. This part is realized by software CompactRIO 9401DI/O module and CompactRIO 9485SSR module: CompactRIO 9401DI/O module monitors hydraulic system power switch signal, control cabinet power switch signal, hydraulic pump station oil pressure, temperature, leakage, liquid level signal status When the status is not normal, the corresponding part of the switch is cut off by the CompactRIO 9485SSR module to ensure the safety of the test.
3.2.3 Processing of data
As shown in Figure 4, the test system has the corresponding NI hardware for each signal acquisition. The CompactRIO 9014's RT and FPGA modules ensure real-time and reliable data acquisition, monitoring, and storage. LabVIEW's formula nodes and XY waveforms provide a good analysis of the data. For the characteristics of long fatigue test and large amount of data, the data is processed using the TDMS data stream in LabVIEW.
Test system field results
4.1 hardware field effect
4.1.1 System function
(1) It is used for the fatigue test of the engine connecting rod and the corresponding evaluation of the test results;
(2) A loading system that can be used as a static load test for engine components;
(3) The static strain gauge can be used to test the stress and strain state of the test piece.
4.1.2 Performance indicators
(1) Loading form: the tension pressure can be continuously adjusted separately, and the load ratio can be achieved;
(2) Pressure range: 0 ~ 28Mpa, manual pressure regulation;
(3) Pulse frequency: ≤22HZ, continuously adjustable;
(4) Loading waveform: asymmetric sine wave;
(5) Control accuracy: tensile and compressive load ≤ 4%, loading frequency ≤ 2%.
4.1.3 Composition
Figure 12: Physical map of the mechanical table
Figure 13: Physical map of the hydraulic loading system
Figure 14: Overall effect diagram of the test system
4.2 software effects
After opening the program, the program main interface is entered. The main interface divides the program into three main modules: test setup, real-time monitoring and data analysis. There are also some information about the test system and the exit system button.
After clicking the "About" button, the basic information interface for the entire test system appears.
After clicking the "Test Setup" button, enter the test setup interface.
The test setup is mainly divided into three parts: test basic information, test system parameters and connecting rod test piece parameters. Setting parameters can be entered manually or loaded into the history. When different test types (stretch static load test, compressed static load test, small head dynamic fatigue test and large head dynamic fatigue test) are selected, the corresponding parameters are displayed for setting.
When you click the "Real Time Monitoring" button, the monitoring interface will appear. When testing the test, the following information is available:
(1) Four waveform diagrams show control waveforms, loading loads, tensile compression loads (optional display), strain waveforms (optional display);
(2) Six indicators show the hydraulic system power switch signal, control cabinet power switch signal, hydraulic pump station oil pressure, temperature, leakage, liquid level signal status;
(3) The time display and the cycle number display are all real-time showing the extent of the fatigue test;
(4) The displacement determination range, the number of strain determinations and the range are all real-time settings for monitoring the fatigue state of the connecting rod test piece;
(5) Initial zeroing eliminates interference when each sensor has no signal, and zeros the cycle to clear the number of cycles;
(6) The frequency and servo valve control signals (positive and negative amplification factors) can be adjusted in real time to adjust the test state;
(7) Real-time display of the peak value of the three-way strain;
(8) The system tensile compression load extreme value shows the currently achievable load extreme value;
(9) The maximum tensile and compressive load of the connecting rod is displayed in real time;
(10) The NI 9263, NI 9205, and NI 9237 timeout indicators show three board timeout statuses.
When you click “Data Analysisâ€, you will enter the data analysis interface, which mainly includes viewing data, starting recording, stopping recording, and input data saving path. During the test, the data can be viewed, and the recording can be started or stopped according to the data condition, and the indicator light is illuminated when the data is recorded.
In terms of data analysis, the LabVIEW formula node and the XY waveform diagram realize the fatigue reliability analysis of the data, and the PSN double logarithmic straight line of the corresponding data can be obtained, as shown in FIG.
Figure 15: PSN double logarithmic fitting straight line
Based on the above design, the test flow of the entire test system is shown in Figure 16.
Figure 16: Test block diagram
V. Conclusion
1. The developed engine connecting rod tension and compression simulation fatigue test bench is the first dedicated test bench with independent intellectual property rights in China.
2. This test bench is based on the test principle that the connecting rod is mainly subjected to tensile and compressive loads. The hardware design of the NI CompactRIO embedded controller and hydraulic loading system makes the test system have high reliability, accuracy and practicability. The software is programmed in the environment, which makes the test system development cycle short and extensible.
3. The test bench adopts asymmetric loading mode to realize the simulation of the actual working condition of the connecting rod; the hydraulic loading PID control realizes the precise control of the load; at the same time, compared with the actual machine test, the strengthening test can be realized, which shortens The test cycle, the test efficiency is improved, and the fatigue performance of the connecting rod can be more comprehensively tested, which has great significance for the design of the new product of the connecting rod and the optimization of the existing products.
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