This type of test is considered as the simplest shock test procedure.
Since electrodynamic shaker use close loop control system, then electrodynamic has almost no issue to generate all type of shock pulse shape.
Most of vibration controller software are able to provide not only HALF SINE, TRAPEZOIDAL and TERMINAL PEAK, but also INITIAL PEAK, TRIANGLE, RECTANGLE and HAVERSINE.
In order to be able to run shock test smoothly by using electrodynamic shaker, there are several items at the software setup that we may need to pay attention:
Acceleration Level
Like sine test, we can calculate if the machine is able to run the test based on the second Newton Law. But for shock test, most of electrodynamic shaker manufacturer commonly put the maximum force or shock test to be twice of maximum force for sine test. This value is generally based on the shock test profile with 6 milli second (ms) pulse duration. Commonly, the force capacity for shock test will vary depending on pulse duration. Maximum force may go as high 2.5 times of the maximum sine force if the pulse duration is less than 3 ms and may go as high as 1.5 times if the pulse duration is more than 6 ms. We should follow the performance curve provided by machine manufacturer.
Pulse Duration
Pulse duration has major influence on the displacement. Longer pulse duration will lead to higher displacement requirement. Most of medium and large size of electrodynamic shaker should be able to perform shock test up to 11 ms pulse duration. Beyond 11 ms, it may still be able to perform the test, but the acceleration level will be limited to a lower level.
Pre-Pulse and Post-Pulse
For running shock test with electrodynamic shaker, pre-pulse and post-pulse is playing important role. Without this setup, it will need very long displacement to achieve the shock acceleration level, and the shaker may not be able to perform the test due to limited displacement capacity. Each vibration controller software has developed their own type of pre-pulse and post-pulse. Other than for reducing the velocity and displacement level, some manufacturers also develop other type of pre-pulse and post-pulse for other purpose, such as reducing frequency content. From another perspective, having pre-pulse and post-pulse means subjecting additional minor shock to UUT before and after the main pulse. Some of test standards allow the acceleration level of pre-pulse and post-pulse to be maximum 20% of the main pulse acceleration level. We may need to ensure that this level of minor pulse is within the acceptable level.
Interval Between Pulses
At some conditions, when the jig/fixtures are we designed, no resonance at low frequency range, we may not need to pay attention to this parameter. Bu sometimes we can't avoid having resonance at relatively low frequency. In this type of case, we may see some additional vibration due to resonance right, after the signal reach the maximum acceleration level. If the interval between pulses is too short, then this may affect the response for the next pulse. We may increase the interval in order to minimize this effect. For test with low number of pulses, this will not affect anything. But for bump test with high number of pulses, this will just make the test period becomes slightly longer
Signal Noise/Distortion
Like performing other test by using electrodynamic shaker, we will also need to pay attention to selection of our control accelerometer, especially when plan to run relatively low acceleration shock test. During start up, some vibration controller software may perform pre-test at 10% of main pulse acceleration level, before going to 100% acceleration level. If our control accelerometer is not sensitive enough, then the noise may become very significant during start up, and it may cause the signal to exceed the abort level. Increasing the start-up level to higher acceleration level will not be our best option as well, since it will subject our UUT with additional shock test (over test). Another signal distortion that may happen is due to lose accelerometer cable connection, due to high acceleration shock level.
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