NV-TECH-Design: Scalable Automatic Modal Hammer (SAM) for accurate, repeatable structural dynamics testing

Transcription

1 NV-TECH-Design: Automatic Modal Hammer (SAM) for accurate, repeatable structural dynamics testing Figure 1: NV-TECH-Design Automatic Modal Hammer (SAM) for structural testing (Patent pending). Modal Testing of Non-Linear Structures To properly optimize the noise, vibration and durability of your products, a good understanding of the structural dynamic properties is required. Structural dynamic properties of assemblies or components are expressed in terms of natural frequencies, modal damping and mode shapes. CAE (computer models) can be used to calculate the structural dynamic properties but these should be validated via experimental modal analysis (EMA) (Figure 2). However, EMA is a procedure that takes structural linearity for granted. Most lightweight structures are joined (welded, glued, pinned) together or made of composite materials with complex geometries, giving them a very non-linear force response. An electromagnetic shaker or a modal impulse hammer are commonly used to excite the structure, and vibration measurements of the responses are made via accelerometers or lasers. However, the application of a shaker can change the dynamics of the structure under test, and Figure 2: Validation of an analytical modal model with the experimental modal model the modal hammer requires a skilled operator in order to collect meaningful data. Both these methods are difficult or impossible to automate for un-skilled operators or end-of-line testing. And although the hammer test has the

2 advantage over shaker testing of not restraining or adding mass to the structure, the force applied manually cannot be kept constant. Although the force is normalised mathematically by the transfer function calculation, the variations in force applied will cause significant inaccuracies in the results, for non-linear structures. As a consequence, if the structure presents a non-proportional force response the simulation will be prone to failures. The newly developed Automatic Modal Hammer (SAM) provides a solution to this problem. Advantages and New Possibilities of the SAM With the NV-TECH-Design Automatic Modal Hammer (SAM), shown in Figure 1, even light-weight structures can be excited with precisely adjustable and reproducible force amplitudes. The SAM is specifically designed in a way that only the inertial mass of the hammer tip impacts the structure, with a finely amplitudeadjustable Dirac impulse. This makes it possible to investigate the non-linear response of composite materials and jointed structures with the SAM, obtaining impact force-dependent natural frequencies and damping ratios. This leads to more accurate experimental data and therefore, a better modal model correlation between CAE and the real structure. If the SAM is used in combination with contactless Laser Doppler Vibrometry (LDV), structural modal measurements can be completely automated. The use of the SAM results in an increased accuracy and higher test effectiveness and practicability. Design of the SAM A variety of competing design objectives had to be considered to solve the technical challenge of applying a precise, repeatable and reaction-free Dirac impulse to a structure. The hammer handle is rigid enough to support the impact tip, but flexible at the same time. Using a unique mechanism (patent pending), only the free inertia of the seismic mass at the piezo-electric sensor impacts the structure. It is essential that the impact remains exactly perpendicular to the surface, even after millions of impacts. Figure 3: Assembly of the SAM Figure 4: Application example of the SAM mounted on a flexible hydraulic arm with a magnetic stand The SAM has a modular design and can be attached to different test stands, to guarantee its precision and reproducibility qualities. It can be attached to a vertical Vernier scale or to a practical hydraulic arm magnetic stand (Figure 3 and Figure 4). Both stands are included with the SAM, and make it simple to setup. The SAM is currently the only alternative in the market offering these capabilities as described in Figure 5:

3 Function principle force velocity idle time 360 measurement electromag. % NO YES NO YES actuator % NO YES NO YES cylinder % NO NO NO YES NV-TECH SAM YES YES YES YES NO Figure 5: Comparison with existing commercial automatic hammers. % means that the adjustment of this parameter is limited or dependent of the adjustment of other variables The Graphical User Interface External control device? Handheld NO NO NO YES - Position, away from the PC work station. This option is useful for larger experimental setups where the PC is not close to the measurement location. The possibility of setting an impact angle other than the default of 15, useful in experimental setups with space constraints. The setting of a large number of impacts. This is useful, for example, in overnight tests with Scanning Laser Doppler Vibrometers, which require many of impacts. Example Results with the SAM Detection of Non-Linear Force/Response Components Figure 7: Example of a non-linear force/response curve. The straight line corresponds to a perfect Hooke-like force/response relationship, while the dotted lines are responses where some degree of non-linearity is introduced. Source: [1] Figure 6: Main interface of the SAM-GUI program The Graphical User Interface (GUI) is a simplified and user-friendly interface, as shown in (Figure 6). The Basic Module of the SAM-GUI program allows full control of the SAM while leaving out the most advanced features some users need. A Pro Module, which unlocks all the program features, is available separately. The Pro Module grants access to the following features: The use of a remote control device to precisely set the hammer tip Zero With the development of the (adjustable force amplitude) Automatic Modal Hammer it is possible to dynamically excite structures with a precise, repeatable force amplitude, at a precisely repeatable excitation location. With this repeatable force level, only one point on a given non-linear stiffness characteristic curve is excited, see Figure 7. This leads to an exactly repeatable response signal from the non-linear structure under test. Figure 8 shows the effect of increasing impact force from 83 N to 1940 N for nonlinear structure (brake-pad). This resulted in a decrease of approximately 27 Hz in the natural frequencies. Only by applying an impact with a precise, repeatable and

4 controllable force level is it possible to study structures with non-linear structural behaviour. This is what the SAM makes possible for the first time. The results can be further used for updating simulation models or for the validation of simulation results. Figure 8: Natural frequency measured on a brake pad. Lighter shades of blue indicate higher impact force amplitudes. The red line shows the natural frequency variation vs. impact force. Source: [2] Repeatability and Reproducibility The repeatability of the excitation force is shown in Figure 9. The SAM provided a variation of force amplitude of 1.65% over the tested force range. This is important for testing non-linear materials such as composite plates. The test was performed on a steel plate with a sampling frequency of 250 khz. Approximately 100 hits per force amplitude were included in the statistical analysis. Figure 9: Boxplot distribution, as a result of 100 impacts on a steel plate Iterative and Automatic Testing Figure 10: Testing a drone rotor blade using 3D SLDV and the SAM. The use of the SAM allows the automated testing of 100+ nodes, each with five averages. Source: [3] The combination of SAM with contactless Scanning Laser Doppler Vibrometry (SLDV)

5 allows fully-automatic, precise and costeffective structural dynamic measurements while keeping testing time to a minimum. An actual test example is shown in Figure 10, where the 100+ nodes are tested automatically. The time interval between impacts can be adjusted to a minimum of 0.6 seconds, which allows tailoring the impacts to the decay time of your structure, with the resulting optimisation of testing time. Scalability The SAM can be provided with several different hammer sizes. This means it can be applied to both small and mediumsized structures. With the larger 2000N hammer, it is possible to precisely test structures such as gearboxes, engine blocks, alternators, generators, turbines etc. The larger version of the SAM is known as SAM2 (Figure 11). Laser Vibrometry, Vol. 8 Conference Proceedings of the Society for Experimental Mechanics Series, pp , Figure 11: Larger Automatic Modal Hammer (SAM2) for the structural testing of more massive structures Picture sources [1] SEPAHVAND, K., Non-linear Modal Analysis, Presentation at the European Modal Analysis Users Group Meeting 2016, Ingolstadt, Germany, [2] BLASCHKE, P. et al., Non-Linearity Identification of Composite Materials by Impact Modal Testing, in WEE SIT, E. et al. (eds.), Sensors and Instrumentation, Vol. 5 Conference Proceedings of the Society for Experimental Mechanics Series, pp. 7-14, [3] ALARCÓN, D.J. et al., Modal Model Validation Using 3D SLDV, Geometry Scanning and FEM of a Multi-Purpose Drone Propeller Blade, in DI MAIO, D., CASTELLINI, P. (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics &