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Vibration and Shock Environmental Test Types

Vibration and Shock Environmental Test Types

Simulated vibration environments in the laboratory are used to qualify products during design, to test products against standards, e.g. Mil-std 810 etc., to locate sources of noise, e.g. in squeak and rattle testing and for stress screening, where the failure modes in components are precipitated. Data Physics SignalCalc analyzers provide a complete measurement capability to support the full range of environmental vibration testing and environmental test applications. The primary test types in shock and vibration testing are discussed below for clarity.

SignalStar Vibration Controllers offer the best-in-class vibration control for all of these shock and vibration applications, and SignalForce Electrodynamic Shakers offer reliable and proven shakers making Data Physics a single source provided for vibration test equipment.

Swept Sine Vibration Control

Historically the first type of vibration test performed, traditional sine sweeps, provide a single sine tone varying frequency, phase, and amplitude. Swept sine vibration tests should test all frequencies, measured or not, by using a continuously varied sine oscillation of controlled amplitude.

Swept sine testing is particularly useful for the study of structural response at resonance. Additionally, structural resonances can be excited at high response levels, making this a good test type for fatigue tests.

Some of the most important features of sine vibration testing are the tracking filters used. High quality digital tracking filters with user selectable fixed or proportional bandwidths should be used to ensure the sine signals are accurately measured and controlled in high noise environments.

Resonance Search and Tracked Dwell

Resonance searches involve sine sweep to obtain a transfer function for evaluation of resonance characteristics. Input parameters such as frequency range, amplitude threshold, and minimum Q factor (quality factor or sharpness factor) are used to determine which modes should be further evaluated or tested.

Resonance dwelling is useful for fatigue testing of many mechanical components. Resonance tracking automatically detects shifts in resonance frequency and adjusts the sine excitation signal to track those shifts precisely. Tracked dwells are common for fatigue testing of high-cycle critical components such as turbine blades and vehicle crank shafts. Learn more about resonance dwell control and one of the most demanding resonance dwell tests, Turbine Blade High Cycle Fatigue Testing.

The SignalStar Resonance Search and Dwell application is extremely capable at dwelling and tracking of structural resonances for fatigue testing. Dwells can automatically track shifts in resonance frequency during fatigue testing and set limits for ending the dwell based on amplitude or total frequency shift.

Multi Frequency Sine Control

Driven by a consortium of German automobile manufacturers, multi frequency sine has been developed as a method of durability testing engine mounted components using multiple simultaneous swept sine vibration testing. The objective of this new specification is to create a uniform method for qualifying components while reducing testing and development costs.

This test technique divides the sweep frequency range into multiple intervals and allows multiple frequencies to be simultaneously active, and considerably reduces the total test time. For example, four sine tones swept simultaneously across individual frequency spans of the same frequency range at the same sweep rate will reduce the time required to produce the same fatigue in a test item by 75 percent compared to a single frequency swept sine test. This method uses the orthogonality properties of sine waves to ensure that a test item has been exposed to every frequency in the sweep range for the required duration of time even though the test itself does not last as long.

The Multi Frequency Sine control software for the Data Physics SignalStar vibration controllers reduces the time taken for sine testing without sacrificing control accuracy and performance.

Random Vibration Control

As vibration testing evolved, methods that better represent real world data were sought out. Random vibration testing does just that by providing statistical confidence with random time data that has an average targeted frequency content and amplitude over the duration of the test. By controlling amplitude and frequency, test data can be correlated to real world data sets.

Driven initially by transportation testing, random vibration data can also be used for aerospace and automotive applications. Applications range from production stress screening of electronic components to prototype testing and qualification of products to military standards. Random testing excites all resonant frequencies simultaneously, and due to this and the statistical nature of random data, it is well suited for vibration qualification tests.

Data Physics has a rich legacy in vibration control that includes many advances in random testing, including the continuous convolution algorithm. Consequently, Data Physics SignalStar Random control capitalizes on advanced control algorithms that provide fast, accurate, stable closed-loop random control.

Mixed Mode (Sine and Random on Random) Control

Mixed mode testing is used to simulate the vibration environment of structures where there is a combination of broadband and cyclic or narrowband energy.

Cyclic energy can be in the form of sine tones (line frequencies) or narrowband random. Examples of Sine on Random (SOR) vibration include helicopter vibration, where random vibration due to turbulence and sine vibration due to rotor blade frequencies are combined. Sine on Random is also frequently used to include vehicle engine vibrations in automotive tests.

Vibration from tracked vehicles is typical of Random on Random (ROR), where narrowband random is superimposed on broadband random. For both sine on random and random on random, the sine and narrowband component frequencies can be fixed or swept.

Military gunfire simulation is another use for mixed mode testing. The controller will provide rapid on/off control of the narrowband signals and simulate gunfire.

Shock and Transient Testing

Classical shock testing on a shaker is an efficient alternative to drop testing, bringing the advantages of better accuracy and repeatability. However, performing shock testing using shakers imposes physical limitations due to the available armature displacement and shaker/amplifier power ratings.

Both shock and short duration transient events are controlled in the same manner as realtime adaptive control. Transient control involves importing signals to reproduce actual measured short-duration data.

SignalStar Classical Shock algorithms have evolved to make the best possible use of available armature displacement and to efficiently handle the dynamics of shock testing with both electrodynamic and hydraulic shakers by using advanced compensation algorithms for pulse synthesis.

SRS Testing

SRS Testing, SRS shock control, or shock response spectrum control, is typically used to simulate the complex vibration environment seen in earthquakes and pyrotechnic shock.

Shock response spectrum is a characterization of the damage potential of a transient waveform. The test reference is specified as a shock response spectrum. A time history (accelerogram) that conforms to the given SRS reference is synthesized. The synthesized waveform is made up of sinusoidal components which are typically either damped sines or sine beats. The synthesis is an iterative process that terminates when the error is smaller than a user specified value.

SignalStar controllers offer advanced synthesis capabilities and reference response spectrum control for easily tackling the most demanding applications.

Time Data Replication

Time waveform replication, or waveform replication, allows for reproducing long duration time waveforms in the lab instead of in the field. This allows for approximating the environment using random or sinusoidal waveforms. For example, by using recorded data from road or flight tests, a more realistic vibration environment can be reproduced, assuring a higher level of quality in the test results.

Time waveform replication does have some drawback when used for qualification testing or in the design process. First, replication will prove that your product or component will work on that given data set, but lacks the statistical variation of random data. It can often be argued that random data is a better real world representation for many cases. Further tradeoffs involve test time. Generally, random testing can be much shorter in duration than time data replication.

Traditional control of replication data uses an iterative technique to develop a drive file (playback file) with relatively open loop or slow control. Leading-edge controllers, such as SignalStar Controllers, have capabilities to perform realtime control over contiguous data blocks.

SignalStar Time Data Replication provides all the tools necessary to take measured field vibration data and reproduce it on a single shaker or multiple shakers in your laboratory.

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