Time-to-angle conversion of measurement data traditionally has been done through labor-intensive and error-prone manual operations where data is exchanged between separate systems. As a result, angle domain testing has been extremely time-consuming and often limited in accuracy. There is a definite need for a better and faster approach. This is why the experts at LMS International have developed LMS Test.Lab Angle Domain Processing as part of the LMS Test.Lab Rotating Machinery solution. Used extensively for vibro-acoustic engine testing, this new module automates the conversion processes and performs all measurement, data exchange and analysis within a single unified system. Integrating the required engine testing capability into a single hardware and software platform significantly shortens the time required for this operation and greatly increases the result accuracy.
One of the first companies to implement the LMS Test.Lab Angle Domain Processing is Federal-Mogul Corporation, a leading global supplier of powertrain components for the automotive industry. At its Plymouth, Michigan, Technical Center, Federal-Mogul is using the new tool predominantly for verifying cylinder system performance – in particular to study piston NVH events in relation to friction optimization and fuel economy.
Challenges in studying NVH events
Federal-Mogul engineers formerly would study piston noise by shuttling data back and forth between separate systems. First, they would program their old Unix-based test system to acquire the required vibration data from accelerometers and strain gauges mounted on an engine undergoing a series of run-up/run-down cycles. Test data was then exported to an internally-developed program for performing the time-to-angle conversion. The results were sent to a script-based system for calculating piston acceleration and other homegrown metrics to identify different types of engine noises. Finally, the realigned data was fed back into the test system for post-processing to create output plots.
“Our old process was very slow,” said Federal-Mogul Test Engineer Dave Thiel. “Several days would elapse before results were available. Then if any unforeseen setup or data acquisition errors occurred, for example, or if more detailed studies were needed in a particular frequency range, tests would have to be completely re-run and results analyzed again in the same cumbersome process. Plus you had to add more time finding and correcting errors in the many manual data conversions required on the different systems. From beginning to end, projects taking several weeks were not uncommon.”
Faster, better NVH tests
LMS Test.Lab Angle Domain Analysis eliminates these problems, since all operations for test data acquisition, time-to-angle conversion, results analysis and post-processing are done in a single smart system. Many critical operations are performed automatically using standard best practice techniques. The system quickly produces output plots including frequency vs. amplitude and frequency vs. cycle plots as well as order tracking and waterfall harmonics plots of frequency vs. crank angle vs. amplitude. From this data, Federal-Mogul engineers can determine where in the duty cycle particular parts of the piston impact the cylinder bore. This gives them a direct link to best resolve the issue.
With extensive filtering and statistical analysis to compare the influence of rotating speeds, temperatures, torques, and various design parameters, engineers can readily compare different pistons, cylinders and operating conditions. They can take all relevant attributes into account to identify which configurations give the best results. For these types of applications, Federal-Mogul uses scatter diagrams with small different-colored tick marks comparing vibration levels during the engine cycle for different conditions – cylinder wall acceleration near top dead center (TDC), therefore directly related to piston mechanical behavior for firing and non-firing strokes for a given frequency range, for example.
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Scatter diagrams let Federal-Mogul engineers compare piston behavior under different operating conditions. The top diagram shows cylinder wall acceleration near top dead center (TDC) for firing and non-firing strokes between 1 and 4 kHz. The bottom plot indicates when vibration exceeds a 10 g-dB limit. |
“Scatter plots comparing piston NVH events at various segments of the engine cycle provides deep insight into noise and vibration characteristics. LMS Test. Lab Angle Domain Processing is a tremendous ‘fix-it’ tool,” said Mr. Thiel. “We weren’t able to do this before we started using the LMS Test.Lab software.” Scatter plots are created with the help of a so-called “gating” feature that allows engineers to zoom in on specific regions of the engine duty cycle for more detailed study – usually where NVH events are most prevalent. Engineers merely specify the range and the system automatically gathers data at a high sampling rate in the angular range of interest. In this way, the system processing resources are focused on determining averaged values from all cylinders which can be compared for the particular portion of the duty cycle. “Gating lets us quickly zoom in on the specific information we need. It is a powerful tool to evaluate engine performance,” he noted.
Value of an integrated platform
According to Mr. Thiel, part of the great value of the LMS Test.Lab Angle Domain Processing output plots is that they are displayed so quickly. “Results are displayed immediately in real time as measurements are being taken rather than making us wait for hours of post-processing,” he explained. “With on-line post processing, we can readily verify the test, correct any set-up mistakes, identify abnormal accelerations immediately, compare run variation plots, focus on areas where greater detail is needed and gain tremendous insight into the root causes of noise problems. This all happens on the spot instead of waiting hours – a significant time-saver. For us, test time has gone from days to minutes.”
High-speed processing is achieved because measurements are made, data analyzed, and output displayed in one single integrated hardware system: the LMS Test.Lab SCADAS Mobile. High-speed data streaming and parallel processing are key to the system’s efficiency. Moreover, a high sampling rate in acquiring data significantly boosts measurement accuracy and resolution. For example, the tacho rate of the LMS SCADAS Mobile with an incremental encoder input card can be as high as 200 kHz – ten times the sampling speed of Federal-Mogul’s former system and far beyond the rate of most conventional systems used in industry. According to Mr. Thiel, this fast data acquisition capability is really helpful when a high data acquisition rate is needed at elevated engine speeds.
In addition, Mr. Thiel likes the easy portability of the LMS SCADAS Mobile unit with its integrated LMS Test.Lab suite of tools for test set-up, measurement, signal conditioning, result analysis, data management and report generation – all in a lightweight laptop-size unit with built-in battery power.
“We can easily carry the unit between our 16 test cells and even perform in-vehicle testing when necessary. The system is convenient to set up, with built-in workbooks, defaults and prompts showing where to enter input values and how to proceed step-by-step through the process. Set-ups are fast with little chance for making mistakes.”
After test completion, the automatic report generation features in LMS Test.Lab further improve the engineering productivity. Whereas days were previously required to manually prepare the necessary documentation, standardized templates with precise test data lets engineers complete critical reports in a few minutes. Thanks to its Microsoft Active Pictures capability, users can simply insert live test data as scalable interactive plots in Microsoft Office tools such as Word and PowerPoint. This way, customers viewing the report do not need to run LMS Test.Lab.
Faster NVH tests. Quicker customer response.
“The ability to run NVH tests faster and more accurately definitely has a positive impact on our group’s business performance,” said Steven Krause, Director of Application Engineering at Federal-Mogul. “Greater throughput translates directly into improved customer response and time to market, with the ability to complete routine jobs more quickly and to accommodate big jobs that otherwise could not be handled. Above all, our group is able to continue improving product quality and value for which Federal-Mogul has been widely recognized in the auto industry for over a century.”
There is certainly no shortage of available NVH measurement and analysis techniques to study engine noise and vibration issues. The global market is full of tools and homegrown techniques to nail the root cause of many engine noise and vibration annoyances, such as engine knock, piston NVH, valve tapping and gear rattle. Most of these are related to the engine duty cycle – specifically, forces exerted by particular components during the intake, compression, combustion and exhaust cycle steps. And in these type of cases, measurements must be converted from the familiar time domain to the angle domain to correspond to the rotational position of the crankshaft.
