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Next generation Transfer Path Analysis to pinpoint noise and vibration problem sources
LMS Engineering Services pioneers new TPA techniques to accelerate troubleshooting throughout the development process
 When new products are developed, one of the constraints manufacturers run into is the difficulty to gain a fast understanding of weak points in the design. Extensive development consultancy experience has allowed LMS Engineering Services to develop analysis methods to overcome this constraint. Advances in testing and simulation technology, for example, made it possible to overcome the constraints of Transfer Path Analysis (TPA) method and allowed LMS to create techniques that can be applied throughout the development process. By using CAE-driven "contribution analysis" concepts, engineers can efficiently identify the root cause of noise problems and quickly evaluate potential solutions.
Avoiding costly design changes late in development
Noise problems mostly surface late in development, when executing design changes is costly and time consuming. Enhanced TPA techniques enable engineers to detect root causes of weaknesses earlier in the development process. Building an experimental transfer path model starts with the identification of operational loads by means of road and chassis dynamometer tests. Subsequently, engineers estimate noise transfer functions, typically in laboratory conditions. The separation into load and transfer specific information is essential in identifying the dominant causes of noise problems and proposing effective solutions. Over recent years, LMS has extended the use of the TPA methodology into CAE through the "contribution analysis" concept which evolved out of the existing unit-force calculation technique. The contribution analysis concept is more powerful as it is based on reliable engineering models, fed with realistic load data sets acquired from experimental TPA. The resulting models can be interpreted in terms of critical problem areas, panels, structural parts, etc., and therefore allow engineers to make a realistic and timely assessment of full-system noise and vibration behavior.
Increasing speed and accuracy of TPA
Recent advances in excitation and measurement instrumentation have contributed to speeding up TPA measurements, while meeting demands for increased accuracy. One application where instrumentation advances have been implemented is reciprocal testing, which is based on the reversibility of the source and receiver due to the reciprocal nature of vibro-acoustic structures. In this field, LMS has developed a range of Volume Velocity Sources (VVS) for specific frequency bands of interests. Recently, LMS has expanded its LMS Qsources excitation hardware portfolio with a new range of inertia shakers that improve the speed and quality of system measurements.
A quick first insight with Operational Path Analysis (OPA)
In analogy with Operational Modal Analysis, an operational equivalent of the TPA technique has been developed,
called Operational Path Analysis (OPA). It expresses a target response in terms of path responses, allowing users to balance each path response such that together, they can be combined as contributions to the total target response. Key to the OPA technique is the calculation of the transmissibility matrix H which is defined as the relation between a single "input" and a single "output" – taken that other inputs are at rest. Through the OPA technique, an assessment of the relative importance of main airborne vs. structureborne contributions can be identified within a single day, compared to one or two weeks with a classic TPA. Although the methodology is very appealing thanks to the reduction of laboratory measurements, one has to be careful to derive "causal" interpretations as from true Transfer Path Analysis. The main outcome of the technique is the path contributions, not the path loads, which renders the technique impractical in combination with CAE data.
Improving speed with Fast TPA
 In many NVH applications, the first challenge is to identify the key contributing subsystems to a particular noise issue. While assessing air-borne versus structure-borne noise, or investigating the impact of front versus rear suspension, NVH teams often do not have the time for lengthy laboratory testing with extensive instrumentation. To address this need, LMS Engineering Services developed a more speedy testing procedure, called Fast TPA. This new TPA approach complements traditional troubleshooting operational measurements with a limited number of additional FRF tests. It reuses existing instrumentation from operational measurements, it involves limited additional laboratory measurements and it does not require removing active systems. Fast TPA is therefore a valid solution for applications where a fast assessment of the main contributing sources or subsystems is required. The increased performance of new types of sensing equipment, such as strain sensors, allows engineers to improve the quality of the path loads without compromising set-up time.
Modal Contribution Analysis (MCA) – covering both NVH and ride & handling
 Path contributions are important when troubleshooting NVH problems. At lower frequencies however, decomposing the global problem in terms of paths becomes less relevant. A similar concept of response contribution can be applied to the analysis of problem sources by using a modal model. In this case, the system functions are represented by the modes, the loads, the modal participations and the operational response for each frequency, representing a sum of modal contributions. Typical applications of Modal Contribution Analysis (MCA) are the assessment of panel modes to airborne noise problems, or the identification of the impact of tire modes on road noise. Besides NVH, the assessment of ride & handling performance also benefits from this technology as it allows low-frequency responses to be broken down into a contribution of modes that represent body weak spots. MCA enables engineers to frontload the identification of these body weak spots which entails important cost savings due to informed decisions regarding choice of materials.
Deploying the right combination of TPA techniques
Through a TPA engineering project, LMS Engineering Services can help development teams to improve their products, and implement countermeasures to quickly obtain better system performance. Dedicated technology transfer programs enable the selection of the right combination of TPA techniques and their deployment in daily development activities. Altogether, the enhanced TPA approach – combining virtual and experimental advances – has put TPA on the map as an intrinsic part of product development. By identifying the main contributors to a particular problem, LMS consultants can efficiently tailor effective design enhancements and efficiently optimize noise and vibration performance.
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