Choosing Accelerometers for In-Situ Testing: From the NASA VIPER Acoustic Modal Experiment

As published at the 34^th Aerospace Testing Seminar, March 31^st, 2025, Los Angeles, CA, USA.

Author: Dale Schick with information from ATA Engineering and NASA Johnson Spaceflight Center.

Dynamic testing of spacecrafts helps to enable mission success, but did you know that different sensors may be chosen for specific tests, even if the type of measurements is the same?

For example, Vibration, Acoustic, and Shock testing all require precise measurement of acceleration. In these cases, the Device Under Test (DUT) is exposed to one of the three environments and its response is the subject of the measurement.

Shock tests require accelerometers that accurately record fast-acting (transient) pulses and therefore need less-sensitive sensors. Accelerometers chosen for Acoustic testing, on the other hand, may be more sensitive so that they can record lower peak-values over longer periods of time.

A Real-World Case: NASA VIPER

Sensors are often chosen for their economy by the test labs or agencies that purchase them for testing, and subsets of them may service many DUT’s over their usable lifetime.

In this note, the NASA Volatiles Investigating Polar Exploration Rover (VIPER) was outfitted with chiefly general-purpose accelerometers for both Vibration and Acoustic testing.

When NASA, Acoustic Research Systems (ARS,) and ATA Engineering prepared to test the rover for Acoustic Launch Qualification with a Direct Field Acoustic Noise (DFAN) system, they saw an opportunity to measure the modal response of the unit in-situ and had to use the general-purpose accelerometers available at the time.

What the Data Revealed

The instrumentation, as mentioned previously, was optimized for DFAN and Vibration testing and was, as such, not ideal for low-level modal testing.

An examination of the Auto Spectra for the entire measurement set reveals an interesting facet of the accelerometer’s capabilities in terms of discernible data above the noise-floor.

Five triaxial accelerometers were able to measure the lateral bending modes of the structure as well as some modes below 30 Hz where the acoustic input begins to roll-off. The five accelerometers consisted of PCB model 356A33 and Kistler model 8763B500BT.

The rest of the accelerometers were Dytran brand model 3133D1T and were only able to capture useful data above 30 Hz for this level of input. The primary difference between the models is the internal amplifier.

In the figure below, the auto spectrum of the accelerometers is shown for those models with a low noise floor (blue) and higher noise floor (red).

Figure 1: Auto Spectrum of Accelerometers used in VIPER acoustic test showing difference in accelerometer noise floor

Planning for In-Situ Testing

When planning for In-situ testing, it may be wise to consider a nominal set of instrumentation to be inclusive of data from Vibration, DFAN, and Modal testing.

In these cases, it is advisable to consider the ultimate measurement capability of the sensor in terms of not only frequency but total range within the bandwidth of frequencies of measure.

Most accelerometer models provide a specification sheet and, though details may vary, a ‘noise floor’ or ‘spectral noise’ measurement may be provided. If the latter is available, considering a lower spectral noise value in the frequency range most relevant to the DUT will provide some benefit.

It’s also worth considering a longer duration exposure to the environment, especially where Modal testing is concerned, to help bring the response above the noise floor. This last bit is the subject of the next application note in this series, so stay tuned!