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Random-on-Random

Superimpose controlled bands of random energy on a random background

There are complex vibration environments where random vibration presents as both random background energy and narrow-band signals. Use Random-on-Random to run a controlled spectrum random vibration test. Random-on-Random is one of the three mixed-mode testing modules in VibrationVIEW.

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Part number 9207

VR9207

What is a Random-on-Random Test?

A Random-on-Random (RoR) test runs a complex test profile that contains two types of random signals: random background and narrow-band signals. This mixed-mode test reflects field environments where the device under test generates a broadband random signal and one or more random “tones.” The narrow-band signals can be swept or fixed frequency and are superimposed on the base random energy profile. The random vibrations are performed simultaneously to simulate a service environment where both modes are present.

Easy Test Entry

Enter frequency/amplitude breakpoints of the background random acceleration spectrum in an easy-to-read, tabular format. The operator can use frequency and amplitude breakpoints or enter one endpoint and the desired dB/octave slope. Over 9,999 separate frequency/amplitude breakpoints can be entered, allowing the entry of virtually any test specification.

Random Narrow-Band Signals

Define up to 32 narrow spectral bands to superimpose on the background random spectrum and sweep between frequencies at a user-programmable rate. The software allows for individuals sweep rates for each random “tone.”

The amplitude, bandwidth, and frequency sweep parameters are all user-programmable. Up to 200 separate frequency/amplitude breakpoints can be entered, allowing the entry of virtually any test specification.

When to Use RoR?

Real-world vibration is inherently random. A random vibration test, then, is a realistic method of bringing a product to failure. A random test excites all frequencies in a defined spectrum at any given time. By exciting all the resonances of the device under test, engineers can determine the interaction between multiple resonances.

An RoR test offers more complexity than a standard random test profile. The power spectral density (PSD) of the broadband random background is superimposed with one or more narrow-band signals, each with individual amplitudes and sweep parameters. The random “tones” are excited simultaneously, which makes RoR tests fitting for product qualification and fatigue testing.

What is the Power Spectral Density (PSD)?

Random VRU Testing CourseRandom vibration is often analyzed with the power spectral density (PSD). The PSD represents the distribution of a signal over a defined frequency spectrum. It reveals resonances and harmonics that may not be visible in a time-history graph.

Normally distributed time-domain data is transformed into frequency-domain data (i.e., a PSD) using the fast Fourier transform. An understanding of the PSD will help engineers to determine the correct parameters to use when generating a PSD from a waveform. To learn more about generating a PSD, please enroll in our Random Testing course on VRU.

Additional Features

  • Configurable Safety Limits: Configurable acceleration limits, line limits, and drive limits can be set by the user to protect the test article and shaker system. The control input is also verified against shaker acceleration limits.
  • Test & Level Scheduling: Tests can be scheduled to run a user-defined length of time, the spectrum level can be scaled by a specified dB-level, percentage, or scaled for a specified RMS acceleration. Level schedules can be entered to run various durations at different acceleration levels.
  • Data Plots: Available graph display options include power/acceleration spectral density (PSD), output voltage spectral density, and channel-to-channel transmissibility. Graphs can be easily auto-scaled, and cursor display can be adjusted. Data and text annotations can be easily placed on the graphs, and data values update live with changes.

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