Vibration in the field is not always solely random or sinusoidal but a combination of vibration types. Mixed-mode vibration tests, such as sine-on-random, replicate these types of complex vibrational environments in the lab. When should an engineer use sine-on-random testing in place of, or in addition to, a sine or random vibration test?
What is Sine-on-Random Testing?
Sine-on-random (SoR) is a mixed-mode vibration test that runs both sine and random vibration. One or multiple sinusoids are superimposed on a random test profile, and the control system outputs the signals to the shaker table at the same time.
Watch: Sine-on-Random Test Startup
In the real world, sinusoidal and random vibrations often coincide. However, the testing industry considers random vibration tests more realistic because most vibration is not predictable like an oscillating sine wave. Therefore, random vibration tests are the go-to for product testing, while sine testing typically has pre-test applications like preventative maintenance and resonance search.
So, why introduce a third option?
A vibration test should always reflect the field environment as closely as possible. There are environments that include significant sine and random vibration. If a product primarily experiences random vibration, but there are sinusoids with considerable acceleration (g) levels, then the engineer should also include the sine tones.
Sine waves oscillate with a consistent frequency and amplitude and typically derive from components with repetitive or rotational motion, such as a pump or rotor. In some circumstances, the sinusoidal vibration is minimal and therefore less important to the overall test. In other cases, the sinusoidal vibration impacts the random background vibration and should be recognized as a necessary part of the test profile.
Sine-on-Random Example Application
All in-service aircraft, including fixed-wing, rotor, or rocket-propelled, have an inherent base random energy. A sinusoidal vibration is also present at various frequencies, typically from the turbine engine or rotor blade.
The quality and longevity of most aircraft components will benefit from mixed-mode testing. Instruments, connectors, batteries, and other conventional components are subject to sinusoidal and random vibrations simultaneously, and these vibrations can result in fatigue and, eventually, failure. As such, SoR is an ideal test method for many aerospace applications.
A helicopter gunfire test is a typical and, perhaps, extreme application of SoR testing. The helicopter is subject to random vibration throughout its flight, and the gunfire introduces a very noticeable high g sinusoidal vibration. Components that have only gone through one test method may, at first, appear to be able to withstand this new environment created by gunfire. However, the mixed vibration environment will fatigue the component faster than the lab test. As the SoR testing method better represents real-world conditions, it is the preferred test procedure.
There are two approaches to consider when creating an SoR test profile. First, you can run the test according to the MIL-STD-810H specifications. This military standard is applicable and easy to use whether the device under test is for a military or civilian application, as many of the test articles are the same or similar.
The second approach is to record field data and then use analysis tools to build a realistic test profile. Modern vibration data recorders have simplified the process of collecting data from the field. However, the subsequent process of data analysis can be complex and time-consuming. To address customers’ difficulties with the analysis of rotational components to create an SoR test, Vibration Research developed the Sine Tracking, Analysis and Generation (STAG) software.
STAG uses order analysis and extraction and sine tone profile creation and acceleration to isolate, extract, and create an accelerated sine profile or series of sine profiles from a recording. It uses the Fatigue Damage Spectrum (FDS) software to analyze the remaining random vibration. In combination with FDS, STAG allows for the analysis, generation, and acceleration of a realistic SoR vibration test.
Sine-on-Random in VibrationVIEW
Using the Sine-on-Random software, engineers can generate sine and random vibrations to simulate the field environment of a device under test where both modes are present. One to 32 floating-point precision sine tones can be superimposed on the random background spectrum and swept back and forth between frequencies at a user-programmable rate. Sine-on-Random is one of the three mixed-mode testing modules in VibrationVIEW.