Classical Shock Testing Control and Analysis Techniques
April 13, 2020
Now that we’ve discussed classical shock pulses and the machinery required to run a shock test, here are several additional techniques that may be useful during testing.
Using Filtering to Determine Velocity and Displacement
Oftentimes, a user will want to determine the velocity and displacement generated during a shock test. If acceleration is the base measurement, additional filters can integrate the acceleration signal into velocity and double integrate the signal into displacement. However, this filtering method can have negative effects on the velocity and displacement values.
When gathering this data, the user will likely need to change the filter settings to minimize the extremely low-frequency content included in the calculation. The low-frequency content will cause variation in the velocity and displacement calculations. When this occurs, the calculations will appear as “wander” in the vs. time graphs.
This low-frequency content is especially visible when multiple pulses are run. The control trace in the displacement versus time graph will have the same shape as the demand line and will begin at the same point but will look offset at some angle. The angle will change between pulses even though the overall shape will remain the same. This indicates that the high pass integration filter needs to be adjusted to remove more of the extreme low-frequency content.
Increasing the cutoff frequency of the integration filter will remove the low-frequency content. This will provide a more realistic representation of the actual velocity and displacement without the effects of the DC offset or other spectral “bleeding” issues. The ability to manipulate the integration filters (and differentiation if using velocity or displacement sensors) is important to consider when investigating different tools for shock analysis.
Shock Tests and High-Frequency Noise
Shock tests can generate a large amount of high-frequency noise, which can be detrimental to the proper analysis of the shock waveform. In many cases, the noise is inconsequential to the relative damage of the product as high-frequency content is often associated with low velocity and displacement.
When the sampling rate used for control and analysis is too low, however, the high-frequency noise can show up as an “aliased” noise. When this occurs, a reflection of the high-frequency noise is visible at the lower frequency multiples. With the VR9500/VR01500 and VibrationVIEW, anti-alias filters are always applied to the signal in the form of a fixed analog anti-alias filter and a variable digital anti-alias filter proportional to the sample rate. If a very low sample rate is used, the anti-alias filter will adjust appropriately.
Conversely, high-frequency data may need to be removed when the sample rate is too high. The high-frequency data may cause noise, rattling, banging, or the relative damage caused by the high frequencies may be inconsequential. Removing this content can be accomplished through the FIR filter (applied after the abort limits are checked) or an IIR filter (applied before the abort limits are checked).
FIR and IIR filters are commonly used to clean up graphs and make them more visually appealing. They also help to meet the test tolerance requirements while sampling at a high enough sample rate to observe high-frequency content.
In VibrationVIEW, the user has the choice of an FIR filter or a series of different IIR filters. These filters have unique characteristics with different effects that can be used to remove frequency content that is outside of the frequencies of interest. Not all shock tests require additional filtering, but, depending on the product, having a system that can apply filtering to the live signal is important for easy analysis.
There are several potential issues—such as spectral bleeding and aliased noise—to keep in mind when running a shock test. It is important to consider these issues when selecting software and other tools for testing.
To access a free demo of Vibration Research’s shock module in VibrationVIEW, click here.