for this experiment is capable of outputting a maximum acceleration of 95g. We can see from Figure 8a that a sinusoidal excitation waveform is amplified incorrectly by the amplifier, in fact, the signals measured from the accelerometer exhibit an increasing level of non linearity with many high harmonics observed in the signal. Figure 7a Response measured at the armature for 7 levels of gain Figure 7b Frequency spectrum of the response measured for the highest gain The maximum capability of the shaker, provided by its specification data, is claimed to be at a level of 95g, 30A and 70V and so the max output power is 2.1kW. A quick experiment for measuring the output power was run to measure the actual power produced for the highest level of gain possible on the amplifier. It was noted that the output levels of the amplifier were 2A and 70V producing an output power of 140W, at the frequency the test rig was tuned for. Such a low output power suggests that the shaker armature was vibrating excessively without producing very much force to the test structure 6 Discussions and improvements We concluded in the previous section that the shaker amplifier was underperforming by outputting a level of power much lower than the maximum achievable. It was understood that poor performance can be caused because of the test rig design, in which the armature resonates at high vibration amplitudes. This mechanical system is capable of producing high responses on the test structure but not using most of the power available. The concept of tuning a test rig proved to be correct but the implementation of it, by using the armature mass as a tuner, proved to be not optimal. Specifically, for testing composite structures which require more power from the shaker because of higher levels of damping as already introduced. The presented mechanical test rig can be used for amplifying the response of the tested component but, in order to do that, the armature of the shaker should vibrate up to the allowable acceleration and in out of phase with the connected rig so as to generate the maximum force. Figure 8 indicates the test rig must be connected to the shaker by a mechanism presenting very high impedance. Another important parameter to take into account during the design of a tuneable rig, is the consequential shift of the resonance of the test structure in the rig. In fact, highlevel vibration produced on the specimen can enhance phenomena of non linearity. When this happens, the resonance of the sample can move to a frequency which is different to the one used for tuning the test rig. Hence, 510
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