Force frequency
Diagrams of the total contact force between the boxes as a function of frequency for linear sine sweep
vibration tests are presented in Figure 3. The peak load in the force‐frequency diagrams corresponds to
the resonance frequency of the system. Changes in the position of this frequency from one test to
another are small and could be due to the sources of uncertainty existing in the tests including friction
loss and hysteresis.
The total contact force in Figure 3 is the sum of the forces registered at all active sensels of the I‐Scan
sensor at a certain time. The contact forces are scaled by the box weight (equal to 190 N). Some
differences are observable between the diagrams for 2–50 and 2–200 Hz. For frequencies lower than the
resonance frequency around 17 Hz, the difference is in the amplitude of the vibration, which is seen to
be higher for the tests with lower sweep rate. It is also seen in the diagrams for the frequency range of
2–200 Hz that, for higher frequencies (over 50 Hz), the force levels decrease significantly to a constant
force level. The sampling frequency of the system was 100 Hz, which, because of Nyquist’s sampling
theorem, limits the information from the higher‐frequency domain. In the sine sweep tests with
frequencies over 50 Hz, results are only valid up to 50 Hz.
It is seen in all the diagrams in Figure 3 that, in the vicinity of the resonance frequency of the
system, the maximum and minimum force diagrams do not present symmetry in loading and
unloading. A possible explanation for this observation is hysteresis in the I‐Scan sensor, i.e. the I‐Scan
system does not react to loading and unloading in the same way and with the same rate.
By comparing the force values obtained from I‐Scan software, it is observed that the initial value
(equal to 1) of the total force from the I‐Scan software scaled by the weight of the box differs from the
value at the end of the test at 50 Hz. This difference is because of the drift in the I‐Scan sensor. This
type of drift is named ‘dynamic drift’ in the present study. Although the boxes were excited with
relatively high frequencies (up to 200 Hz), it is seen in Figure 3 that there only exists one observable
resonance frequency in this range. As our linear sine sweep tests were carried out with a constant
acceleration of 0.5 g, the amplitude of the vibrations decreases with frequency. Hence, the second
resonance frequency of the system could probably be buried in the parts with low amplitude.
Figure 3. Scaled total contact force versus frequency diagrams from linear sine sweep test results. The
results to the left and the right are for the test according to (a) 75S50 and (b) 75S200, respectively.
Each test took 2 min to complete, and the resonance frequency is around 17 Hz.
322 A. JAMIALAHMADI, T. TROST AND S. ÖSTLUND
Copyright © 2011 John Wiley & Sons, Ltd. Packag. Technol. Sci. 2011; 24: 317–329
DOI: 10.1002/