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Figure 5.2.: Impact surface with different inclinations. The target is named according
to the inclination θ as marked in the left photo. The impact surface
was 2 mm wide. The field of view of the imaging system was close to
the top edge of the target.
acceleration was reduced from 80g to less than 1g at the rotational frequency of
20 Hz, which was close to the resonance frequency. In practice, the vibration was
minimal. The balance test was done without the targets. In order to keep the dynamic
balance the weight difference between each pair of targets were limited to
0.01 g. Besides the weight of the target was around 3.4 g, minimal compared to
total weight of the entire rotator, approximately 1 kg.
The second risk was the fatigue of material. The rotating targets exerted centrifugal
force on the rim of the bicycle wheel. The force was unsteady because of
continual on and off operation of the motor and the adjustment of the rotational
frequency. Therefore the rim was subjected to a cyclic loading which in general creates
progressive and localized structural damage of a material. It was tested that
the wheel with targets was able to sustain the maximum rotational frequency of
50 Hz, but the nominal maximum stress for fatigue is less than the ultimate tensile
stress limit, and may be below the yield stress limit. It was unknown when the rim
would break. Therefore 2 mm thick aluminum plates were mounted on the frame
in order to contain the risk in a limited space. After operation of three years, the
problem with the material fatigue did not emerge.
It is unavoidable that strong wind was caused by the fast rotating target. The
small drops were blown away from the region of observation. Because of this limitation,
the maximum rotational frequency applied was set as 30 Hz, corresponding
to a tangential speed of 63m/s. The wind was recognized as one of the two major
factors which led to low probability of successful capturing of the impact process.
128 5. Experimental Setup