Master Thesis - OUFTI-1

tip, but this one is subject to the "bounce" eect, which can cause perturbations on the
vibration response of the structure. No additional mass was used to increase the hammer
impact. Note that an exponential window was applied to the excitation signal. This is a
current practice when a hammer is used to perform the test.
It should also be noted that no impact averaging was performed. Indeed, to apply this
method, impacts include in a series must be the same and, any dierence in the impact
location or intensity has a signicant eect on the results. Now, the OBC 2 card is so
small that the reproduction of the same impact several time can not be assured. In our
case, the excitation point was chosen on a corner of the card (see Figure 4.15).
Acquisition system
Finally, the acquisition system, which takes measurements on the structure, has to be
chosen. An important requirement, that will be discussed in section 4.3.8, is that this
acquisition system does not interfere with the dynamic response of the structure.
In our case, it was decided to use an accelerometer. This type of sensor is currently
used when the signal to measure is a high-frequency one (e.g., a shock), which is the case
here. Once again, owing to the restricted dimensions and mass of the electronic card, this
accelerometer has to be as lightweight and small as possible, to avoid any interference with
the OBC 2 dynamic response. In our case, the accelerometer used weights only 0.2 g and
is enough small for being placed everywhere on the electronic card.
It was chosen to realize 22 measurements on the card. According to this particular
choice, the best solution to attach the accelerometer is the beeswax, which allows a good
grip without interfering with the dynamic response, and an easy handling. The measurement
points are presented in Figure 4.15.
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