Across the Picket Fence: Influence of Sampling Frequency in Automatic Impact Modal Testing

  • Daniel J. AlarcónEmail author
  • Sarah Schneider
  • Robert Kamenzky
  • Peter Blaschke
Conference paper
Part of the Conference Proceedings of the Society for Experimental Mechanics Series book series (CPSEMS)


Automatic impact modal testing is a technique gaining momentum in recent years thanks to the popularization of Scanning Laser Doppler Vibrometry. These systems allow automatizing the output measurement of thousands of degrees of freedom in a short time. The use of automatic impact modal hammers allows automatizing the excitation input and broadband excitation without loading a structure with an extra mass or other drawbacks. However, the impact force repeatability is a prominent concern among test engineers, especially those who work with materials with non-proportional force/response ratios. Assessing the impact force repeatability of a given automatic modal hammer or test rig is necessary in order to ensure the right response level is measured impact after impact.

The assessment procedure can be misleading if not done right. Studying the automatic modal hammer repeatability under typical modal test conditions invariably leads to impact signals strongly distorted by the so called picket fence effect. This results in impacts sampled by only 3–4 data points; insufficient to accurately describe the actual impact force signals and the short contact times between hammer tip and structure. In the reality, the impacts are of larger magnitudes and shorter contact times than what is shown by the analyzer in typical test conditions.

This work studies the influence of the sampling frequency and the test structures used on the repeatability assessment of automatic impact modal hammers. Impact force signals are acquired in this work with enough resolution to eliminate the picket fence effect and truly evaluate how repeatable and reproducible automatic impacts are. The practicality of the procedure, which involves very large datasets and long testing times, is discussed. Guidelines are offered at the end of the paper for a successful repeatability and reproducibility assessment of automatic impact modal hammers.


Experimental modal analysis Sampling Automatic modal hammer Impact modal testing Validation 



Degree of Freedom


Fast Fourier Transformation


Frequency Response Function


Laser Doppler Vibrometry


Repeatability and Reproducibility


Scalable Automatic Modal Hammer


  1. 1.
    Sauer, J., et al.: Modal Shapes Extraction by Automated 3D Laser Scanning Vibrometry with High Spatial Resolution for Detailed FE Model Validation, Oral-only presentation at the EuroBrake 2017 Conference #EB2017-VDT-024, May 2017Google Scholar
  2. 2.
    Ritzmann, S., Meyeringh, M.: Ermüdungstests an Triebwerksbaugruppen unter speziellen modalen Bedingungen, Proceedings of the 4th VDI Conference in Vibration Analysis and Identification, VDI-Berichte 2259, p. 149, March 2016Google Scholar
  3. 3.
    Keil, M.: Automatic Shim Damping Tests under Different Conditions and Setups – Comparison of Different Damping Estimation Methods, Oral-only presentation at the EuroBrake 2017 Conference #EB2017-VDT-017, May 2017Google Scholar
  4. 4.
    Rausch, C., et al.: Identification of Brake Disc Material Properties by Specimen Damping Measurement, Proceedings of the EuroBrake 2017 Conference, #EB2017-VDT-022, May 2017Google Scholar
  5. 5.
    The Modal Shop, Inc.: Impact Hammer Calibration System Models 9155D-961. Online resource [Retrieved on 20th October 2017]
  6. 6.
    Avitabile, P., Peres, M.: Excitation Techmiques – Do’s and Don’ts, Presentation at the XXX International Modal Analysis Conference, February 2012Google Scholar
  7. 7.
    Avitabile, P.: Someone told me that you can’t accept a FRF when the input spectrum has more than 20 dB rolloff SEM Experimental Techniques, pp. 1–2, October 2011Google Scholar

Copyright information

© The Society for Experimental Mechanics, Inc. 2019

Authors and Affiliations

  • Daniel J. Alarcón
    • 1
    Email author
  • Sarah Schneider
    • 1
  • Robert Kamenzky
    • 1
  • Peter Blaschke
    • 1
    • 2
  1. 1.Technical University of Applied Sciences WildauWildauGermany
  2. 2.Noise-Vibration-Technology GmbHSteinheim a.d. MurrGermany

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