Classical Shock Tests on Shakers
April 13, 2020
There are many considerations when selecting the appropriate shock-generating device for a product. A shaker can reliably generate many pulse shapes and, for a routine shock test, can be a cost- and time-efficient option.
However, there are a few caveats to running a shock pulse on a shaker. When classical shock pulses are generated by a shock machine, the table is arrested by a large seismic mass, brakes, gas pistons, or some other mechanical method. Vibration shakers do not have mechanical stops for this purpose.
Additionally, shakers can generate only a fixed amount of velocity and displacement. To address this limitation, the shaker controller employs a process called “compensation.”
Compensation Pulses in Classical Shock Tests
Classical shock pulses are unipolar acceleration pulses. The goal of running a classical shock pulse on an electro-dynamic shaker is to generate the same, or similar, delta-velocity as a shock machine. Classical shock pulses are intended to replicate the effect of a shock machine where the primary characteristic being evaluated is delta-velocity.
In order to run a classical shock test on a shaker, the pulse must start and end at zero acceleration, velocity, and displacement. Pre- and/or post- compensation pulses must be added to drive the shaker back to these zero parameters.
Without the compensation pulses, the velocity and displacement won’t be arrested by the generated signal. Continued velocity and displacement can be dangerous and cause expensive damage.
The addition of compensation pulses changes the frequency content generated by the pulse; however, there are many benefits to running classical shock pulses on a shaker system. For the most part, the pulse can be run in a positive and negative direction without re-orienting the product. Therefore, the product only needs to be re-oriented three times to excite all six orthogonal axes per the requirements of most shock standards. Additionally, the pulse is repeatable, which makes analysis and comparison easier and reduces measurement uncertainty caused by human variables.
Requirements for Compensation Pulses
In the paper titled “Shock’n on Shakers (PDF)” published in the Sound and Vibration Magazine, George Fox Lang details the requirements for compensation pulses in classical shock tests.
Most importantly, compensation must be applied when running a unipolar pulse on a shaker. Compensation pulses can be added before or after the test pulse and, in general, should include two pulses of opposite sign.
The amplitude of the pulses can be lower than the standard definition and they do not need to be equal. However, the lower the amplitude of the compensation pulses, the longer the total duration of the standard pulse. This can pose a controller problem.
Additionally, the compensation pulses do not have to be the same shape as the standard.
Importance of Compensation Pulses: an Analogy
The following analogy further explains why we need compensation pulses.
Imagine a car stopped at a stop sign. In its current state, the car has zero acceleration, velocity, and displacement. When the car accelerates, the velocity and displacement, or total distance from the starting point, increases.
After putting the pedal to the floor—thereby reaching max acceleration—the driver slowly takes his foot off the pedal. Assuming there is no negative acceleration brought on by friction, wind resistance, etc., the car has now reached zero acceleration. Although the driver has stopped accelerating, the car is still moving at a constant velocity and its displacement is increasing. This analogy reminds us that zero acceleration does not necessarily also mean zero velocity and displacement.
Without compensation pulses, a similar situation would apply to the armature of the shaker. When the acceleration returns to zero, the velocity and displacement will continue to increase until the motion of the shaker is arrested. The mechanical stops of the shaker are not designed for this and as it would introduce significant additional shock into the product. In the end, this action would most likely cause significant damage to the shaker.
Holes in the Half-Sine Frequency Spectrum
The half-sine pulse is primarily used to excite mechanical resonances. The main issue with half-sine pulses is the large holes in the frequency spectrum, which are caused by the shape of the pulse.
These holes have less definition if run on a shaker because of the effect of the compensation pulses on the overall frequency response. However, they are still visible. If a product’s resonance lines up with one of the holes, very little acceleration would be driven into the resonance and there would be minimal excitation.
If we remove the pre- and post- compensation pulses and then reanalyze the time waveforms, another issue arises with the half-sine pulse. The “holes” in the frequency spectrum become more defined.
By changing the pulse duration, these holes can be shifted higher and lower in frequency. The location of these holes must be considered when running a half-sine pulse. If the resonant frequency of the product falls within the bandwidth of these holes, the frequency will not be excited. In such cases, the test will be ineffective and the product not properly tested.
These holes are another reason why more test specifications are moving away from the half-sine pulse and toward the terminal-peak pulse shape. The terminal-peak does not have the same gaps in the frequency spectrum and better excites the resonances of the product being evaluated.
Running Shock Tests on a Shaker
There are conveniences when running shock tests on a shaker, but also some drawbacks. It is important to remember that classical shock pulses are not a representation of the real world. Rather, they are designed as an evaluative tool and help us observe how resonance is amplified by a transient of a defined length.
Additionally, the compensation pulses will distort the frequency response of the system. Figure 2.1 displays the energy spectral density (ESD) for a terminal-peak waveform with and without compensation pulses. The change in the frequency response is created by the addition of the pre- and post-compensation pulses.
There are methods to help minimize the amount of frequency content generated by the compensation pulses. In the end, however, there is always a trade-off when running a test on a shaker rather than another mechanical shock machine without the same limits.
There are standards changing the tolerancing requirements for pulses and extending them into the time duration with the pre- and post-compensation pulses. The change in tolerance requires lower amplitude pre- and post-compensation pulses which extends the required total time duration and increases the overall velocity and displacement of the test. The primary goal of this change is to make sure the delta-velocity of a test generated on a shaker is similar to that of a mechanical shock machine.
There are many applications for running classical shock tests on electrodynamic or servo-hydraulic shakers. With a shaker, you get the convenience of applying positive and negative pulses without re-orienting the product, the repeatability of the waveform shape, and the ability to use common fixturing to other vibration tests. However, the limitations of a shaker must be taken into account to make sure the test is still meeting the requirements of the test standard.