Vibroacoustic tests enable engineers to quickly detect anomalies and failure points in machinery and devices, as part of an initiative known as predictive maintenance that helps companies reduce equipment downtime while increasing efficiency.
NASA-STD-7001 defines mission-specific acoustic requirements for spaceflight payload hardware. The standard includes qualification, acceptance and protoflight vibration tests with random vibration levels that reflect flight conditions.
Random Vibration Testing
Random vibration testing is one of the primary methods used for conducting vibration tests, alongside sine vibration testing. Using closed-loop feedback control of a shaker table or excitation system to generate random vibration profiles reproduce mechanical motions which occur naturally such as aircraft on runways and delivery trucks on rough roads; producing harmonics, intermodulation distortion and noise frequencies which occur at random intervals along their spectrum of frequencies with their associated peaks and valleys at random intervals.
Random vibrations are managed through a computer program, which analyses measured data to calculate a peak-and-valley frequency distribution index (PPD), which engineers compare against the standard deviations for each amplitude level to ascertain if random vibrations can be tolerated by hardware under test.
NASA-STD-7001 mandates random vibration tests as a measure to reduce the probability of flight-worthy hardware failure and ensure compliance. This method is particularly helpful in detecting workmanship defects in electronics assemblies as well as evaluating design and structural integrity for components, subsystems, or complete satellites.
Random tests may be conducted using either one or multiple random test profiles, depending on the needs and environment of the hardware being tested. Engineers can easily save and reuse an old profile by clicking a button.
Engineers using random tests employ a computer program to generate a random vibration profile that closely corresponds with expected environmental conditions of the hardware being tested. This program could extrapolate from known payload environments or utilize statistical energy analysis techniques for prediction of vibration response of hardware under test.
The test system then runs the profile, feeding it to a shaker controller that drives a shaker table to replicate the selected random vibration pattern. A vibration controller monitors g2/Hz PSD levels, and when they exceed certain tolerance thresholds it interrupts testing to prevent vibrations from exceeding tolerance levels; once complete the PSD data can then be reviewed to identify and resolve any issues found during testing cycles.
Reverberant Vibration Testing
Reverberant tests involve sound waves bouncing off interior surfaces of a room and creating an undesirable acoustic environment for testing. Volume and total absorption from interior walls, ceiling, and floor contribute to this long reverberation time which should be measured using an echo chamber if possible, but an acoustic generator also gives approximate measurements.
Standing wave method offers more precise measurement of space reverberation times than any other. Using this approach requires using a microphone with precise placement near a specific surface in the test area and plotting their standing waves against graph paper in order to calculate reverberation time.
Vibration testing is often employed to reevaluate the dynamic response of spacecraft hardware that has been modified during manufacturing or assembly, and is frequently performed on CubeSats due to their unique structural properties and configuration.
At minimum, vibration tests verify that a spacecraft can withstand launch conditions. They also can identify any manufacturing or design flaws which might arise as well as hidden issues which require further investigation – these latter tests are often referred to as workmanship tests or “qualification model” evaluation.
Sometimes a spacecraft requires more extensive inspection. This is especially the case for high area-to-mass ratio structures that can easily become fatigued by high intensity acoustic levels. NASA Technical Handbook’s Direct Field Acoustic Testing methodology may offer an effective and simpler alternative than testing in a reverberant chamber.
To conduct this type of testing, a Team Corporation MK VI (150,000 W) or MK VII (200,000 W) Acoustic Generator is employed. These machines create high-frequency vibration by means of 23 Team Acoustic Modulators connected to NASA’s Space Environment Complex (SEC), creating high frequency vibration. Together these powerful instruments can produce 163 decibels of noise to immerse test articles with.
Direct-Field Vibration Testing
Vibration is a natural occurrence that occurs whenever an object encounters forces that push it beyond its equilibrium points and out of resting or equilibrium positions. Vibrations caused by such forces may take the form of highs and lows or back and forth swings depending on their frequency of application.
Engineers use vibration testing equipment to simulate and assess the effects of various vibrations on components, materials or structures within products. By analyzing the data generated during such tests, engineers can identify weaknesses, structural flaws or design flaws which could cause failure or damage during use.
Vibration testing allows engineers to uncover any vibration-related problems within a product, including excessive or uneven loads, structural imbalances or lack of adequate insulation and dampening materials. Vibration testing also serves as a powerful way of gauging how new or revised designs affect product performance and durability.
Applying vibration testing procedures that follow established industry standards helps to ensure repeatable and reliable results. A variety of organizations set vibration test standards across industries and applications – ISO, ASTM, MIL-STD, and SAE being some of the more popular ones.
As part of a thorough vibration test, engineers use software to analyze each vibration scenario. This analysis helps them pinpoint any issues which prevent products from functioning as intended or meeting desired applications.
One popular analysis technique is Fourier Transform, which decomposes time-domain signals into their frequency components and can help identify an object’s dominant frequencies, harmonics and other spectral features. Furthermore, Fourier Transform serves as the basis of power spectral density (PSD) plots which show energy distribution among different frequency bands of vibration signals.
Engineers looking to conduct random vibration testing more precisely may employ a g2/Hz PSD curve as an effective measure. This metric indicates the average acceleration per hertz (Hz) across an frequency range; its shape defines both test profile and spectrum – an indicator of vibration force exerted on a device during testing. Engineers can also measure displacement or velocity to gain further insight into dynamic properties; velocity measures how fast something moves during a particular period while displacement emphasizes lower frequencies.
Small Satellite Vibration Testing
Small satellites must withstand an array of vibrations during launch and space flight, potentially exposing structural flaws that compromise payload hardware integrity. Luckily, advanced vibration testing technologies enable teams to optimize their designs while mitigating risks.
Vibration tests enable engineers to gain an insight into how their designs react to forces across a spectrum of frequencies, helping them identify resonance modes and amplitudes that may lead to structural damage, such as connector loosening or PCB cracking. Additionally, vibration tests may prevent premature failure due to miscalculated fatigue loads.
Vibration test methods for small satellites are extensive. Vibration-testing software enables fast preparation of test profiles and analysis of results, and highly customizable settings enable users to tailor test settings specifically to their applications; such as setting maximum expected flight level (MEFL) levels or creating PSD profiles via closed-loop control in multiple axes.
Small satellite vibration testing typically begins early in the design process, when specifications are drawn from historical records and empirical evidence. As the project evolves, however, these parameters must be revisited based on new information or changes to design objectives; at this stage random vibration testing in a reverberant chamber may provide an ideal method of evaluating equipment survivability.
As demand for vibro-acoustic testing services increases, more scalable and distributed shaker systems are emerging to support CubeSat development. These setups can accommodate the volume of an entire 12U CubeSat assembly for quick testing without spending a fortune in time and expense. Furthermore, digital modeling tools and simulation environments have become an indispensable resource in bridgeing the gap between physical testing and late-stage model verification – ultimately helping CubeSat developers construct robust spacecraft designed for launch.