To perform realistic durability tests in the lab, our customer relies on data gathered from the physical tests on the track. The track surface is a complex topography, realistically reproducing the vehicle responses that drivers experience through a lifetime of hard highway miles, or long stretches of unimproved roads. The test vehicle, equipped with various sensors, captures data from accelerometers, bridge/strain gauges, wheel force transducers, displacement sensors, and voltage measurements.
Although the number of channels varies depending on the type of vehicle, the minimum number of channels required to drive a shaker test is equal to the number of actuators used to drive the test. In the case of our customer’s cab shaker, a minimum of seven channels are required to produce a road signal through iteration to replay in the lab. The cab shaker has four vertical inputs, two horizontal or lateral inputs, and one fore-aft input. This allows for six degrees of freedom (DOF) on the cab shaker excluding torsion. However, seven channels do not always provide enough information to accurately describe the response of the vehicle on the test bench compared to the track.
The test team typically gather data from approximately 50 different sensors shared between the test track and the shaker lab. The more accurate the data, the better the simulation result when iterations are complete in the shaker lab. The SomatXR data acquisition equipment gathers data from these channels while a test driver pilots the vehicle on the test track. This phase takes about two weeks to complete.
Once the test data has been collected, it is then analyzed using HBK’s GlyphWorks data processing system, which contains a comprehensive set of standard and specialized tools for durability analysis. Designed to handle huge amounts of data, GlyphWorks allows the engineers to visualize, analyze, and manipulate the test data, helping identify the most significant parts of the road test data. This selective data is then used to refine the drive file for lab testing.
Once the track data is trimmed, it is placed in the shaker controller and loaded into Instron’s road signal software, TWR. TWR handles all road signal iterations and specimen response characteristics such as white/pink noise models.
To refine the drive file, the engineers instrument an assembly, such as a truck cab, with sensors installed in the same positions as those for the test track vehicle. They then feed the drive file to the Instron controller, run it, and measure the response data at each sensor location.
Using TWR and GlyphWorks processes, the test engineers can determine the iteration quality. Typically, pseudo damage calculations are performed in GlyphWorks to ensure that the vehicle or test specimen is receiving the appropriate damage targets in the test lab just as it would at the proving ground. When damage values and RMS error converge to a constant, the final iteration is analyzed for conformity across the vehicle to the test track. To get to this point typically takes between 12 to16 iteration steps.
Reasonable value is based on a judgment call from the durability test engineers based on understanding the differences between the full vehicle and chassis. The Instron TWR software blends RMS error and pseudo damage calculations on the transducers with a fixed intercept and slope. The test engineers also use level crossing comparisons to judge if the right number of zero crossings occurred during the test, and at what amplitude levels are captured. This varies based on the test intent – a simple hood test and a test on the chassis frame are treated differently.
At this point, the team is ready to start durability testing using the final drive file. Once a test has been run, they use GlyphWorks to analyze the test data. And, when the data is ready to be archived, they use nCode Automation that not only eases test data storage and retrieval, but also makes it easier to share test data with design engineers.