Developing an Engine Connecting Rod Fatigue Test System Based on CompactRIO

Author(s):

Hongrui Liu - Automobile Engineering Research Institute, Department of Energy Engineering, Zhejiang University
Zhentao Liu - Automobile Engineering Research Institute, Department of Energy Engineering, Zhejiang University

Strength, stiffness, and fatigue are key in determining construction machinery component reliability. The connecting rod is the core component of an internal combustion engine with a reciprocating piston, and it must withstand a large alternating load. Therefore, connecting rod reliability directly affects internal combustion engine operational safety. Approximately 60 to 90 percent of connecting rod damage is caused by fatigue, and fatigue damages are mainly caused by dynamic alternating loads.

The main methods for studying connecting rod durability include modeling, in-field tests, and simulation durability tests. Modeling is convenient, fast, low cost, and popular. In-field tests can reveal connecting rod working conditions, but the test cycle is long, costly, and cannot be reinforced. Simulation durability tests are efficient, have short cycles, and can be reinforced to comprehensively test connecting rod fatigue.

Connecting Rod Background and Design Principles

As shown in Figure 1, connecting rod movement is complicated when the engine is running. The smaller end is reciprocating, the bigger end is rotating, and the rod is moving on a plane. Meanwhile, the intricate connecting rod forcing situation includes two parts: a gas explosion and reciprocating piston inertia, and the inertia of the rod itself, including reciprocating inertial force, centrifugal force, and lateral bending moment, which is relatively small.

Test System Design

The test system, based on LabVIEW and a CompactRIO embedded controller, performs hydraulic loading control; data acquisition, storage, and analysis; and safety monitoring.

Figure 2 shows the four-column mechanical frame design. It fixes the connecting rod and supports the connecting rod fixture, rail, moving plate, and reacting plate.

The hydraulic loading system, shown in Figure 3, simulates a predefined load for the fatigue test. We developed the control system using LabVIEW; an NI CompactRIO Controller; an NI-9237 simultaneous bridge module; an NI-9205 analog input module; an NI-9263 analog output module; an NI-9401 bidirectional I/O module; and an NI-9485 solid state relay (SSR) module.

The CompactRIO Controller is reliable and suitable for real-time operation, and it makes it easy to perform strain, tensile, and compressive load signal measurements and servo valve control output. We used an NI 9237 simultaneous bridge module to acquire the strain and connecting rod load signals. The strain gage is attached to a rod, and the NI 9237 module acquires the strain signal while the interface load cell acquires the load signal.

An NI 9205 analog input module acquires the pressure signal in two cylinder cavities through a Kistler pressure transducer. An NI 9263 analog output module streams the predefined pressure signal to control the hydraulic servo valve for asymmetric loading. An NI 9401 bidirectional I/O module monitors test system status, including the hydraulic system power switch signal; control cabinet power switch signal; oil pressure; temperature; liquid level; and hydraulic pump station leakage signal. The indicating lamp lights up when corresponding faults occur. Lastly, an NI 9485 SSR module switches the faulty part off to ensure safety.

We developed the test system software with LabVIEW system design software for hydraulic loading control, test condition monitoring, and data processing.

Asymmetric Sine Wave Loading Method

Asymmetric loading is performed based on the FPGA memory in the CompactRIO Controller and the NI 9263 module. The FPGA can record a predefined quantity of values and record the address of each value sequentially. The NI 9263 module outputs voltage waveform to control the servo valve. The positive and negative polarity of the output voltage is in accordance with the tensile or compressive load, respectively, and the voltage amplitude controls the servo valve opening and changes the tensile or compressive load amplitude. Figure 7 shows the asymmetric waveform using the connecting rod.

PID Load Control

Proportional integral derivative (PID) control is mainly used for dynamic fatigue testing. To ensure the actual load reaches a predefined maximum tensile and compressive load value during nonsymmetric tension-compression load, the maximum tensile and compressive loads are set as the PID control target value. Meanwhile, the test system acquires the actual load signal on the rod specimen and the maximum value is set as the PID control feedback (Figure 8).

Figure 9 shows the PID control code developed with the LabVIEW FPGA Module, which greatly simplifies the control process.

Figures 10 and 11 show load PID control simulation. Figure 10 shows PID control configurations. After setting the PID module parameters, such as changing the setpoint from 0 to 14.3, we obtain a response curve, as shown in Figure 11. When the time unit is 50 ms/bin, and the setpoint (red) significantly changes, the process variable (blue) achieves the setpoint and remains stable in 3S. The simulation results show that the load PID control meets test system requirements. Meanwhile, we can change the response characteristics by adjusting PID parameters.

Test Status Monitoring

Test status monitoring includes fatigue damage, load, and safety conditions. The NI 9237 module acquires the connecting rod strain for fatigue damage monitoring. The real-time strain waveform and peak-and-valley value display in the control panel. When the strain exceeds a predefined time limit, the rod is defined as damaged and the test pauses automatically for operator load inspection.

Load Monitoring

Load monitoring includes connecting rod load monitoring, and cylinder upper and lower cavity pressure monitoring. An NI 9201 module acquires these three load signals and displays them on the monitor in real time

Safety Monitoring

Because guaranteed safety is essential during test, NI 9401 and NI 9485 modules monitor the parts susceptible to fault occurrence and perform fault handling. An NI 9401 monitors the hydraulic system power switch signal; control cabinet power supply switch signal; hydraulic pump station oil pressure; temperature; leakage; and liquid level in real time. When faults occur, an NI 9485 powers down the corresponding part to ensure safety.

Data Processing

As shown in Figure 4, the test system acquires each signal with the corresponding NI hardware module. LabVIEW Real-Time and LabVIEW FPGA running on the CompactRIO Controller guarantee real-time data acquisition, monitoring, and storage performance and reliability. We use the LabVIEW Formula Node and XY waveform chart for data analysis. Considering that fatigue tests are lengthy and generate a large amount of data, we use Technical Data Management Streaming (TDMS) for data processing.

Field Test System

Figures 12 through 15 show system hardware and software components.

A test setting includes basic information, test configurations, and connecting rod parameters. We can manually input set parameters or load them from previous settings. When selecting among different types of tests, the corresponding parameters display (Figure 12).

Pressing the data analysis button displays the data analysis interface (Figure 13). During the test, we can view data and start or stop data recording. The LED indicator is lit during data recording.

We can use the LabVIEW Formula Node and XY chart to analyze fatigue test data and generate a survival rate, stress-strain, and fatigue life double logarithmic fitting curve (Figure 14).

Figure 15 shows the test procedure block diagram.

Conclusion

This test system is the first engine connecting rod tensile and compressive fatigue test rig developed independently with intellectual property in China. With a CompactRIO embedded controller and hydraulic loading system hardware, it is highly reliable, accurate, and practical. Using the LabVIEW development environment, we developed the test software within a short time and can easily expand it in the future.

We can use the system to perform reinforced tests and shorten the test cycle, improve efficiency, and fully inspect connecting rod fatigue characteristics. The test system is significant for connecting rod product design and optimization.

Author Information:

Hongrui Liu
Automobile Engineering Research Institute, Department of Energy Engineering, Zhejiang University
China
liuhongrui_zju@163.com