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Agenda• Why should we care about vibrations ?• Where do vibrations come from ?• What are the types of vibration and how are theyanalyzed from a test lab perspective ?• How are vibrations replicated in the lab ?• In Practice• Questions?
Why should we care about vibrations ?Although some specific applications aim at creating‘good’ vibrations (loudspeakers), most vibrationsources have the potential to create issues.
Why should we care about vibrations ?(cont.)Issues can be of different kinds:• Functional (CD skip)• Comfort (squeak, rattle)• Structural
Where do vibrations come from ?• Machinery• Road
Where do vibrations come from ?• Wind load (Tacoma bridge)• Ground motion
What are the different types of vibrations?• Sinusoidal vibration• Most basic• Simple motion• One (1) frequency present at any given time
Sinusoidal Vibration - Quantification• Typically expressed as:• Acceleration vs. frequency• Velocity vs. frequency• Displacement vs. frequency• Any combination of the above vs. frequency
• Random vibration• ‘Random’ nature due to incapacity to predictprecise vibration level at any given time• Quantified using statistical tools• Broadband, multi-frequency content• More closely match real-world excitationsWhat are the different types of vibrations?
Random Vibration – Quantification• Expressed as PSD (‘Power Spectral Density’) or ASD(‘Acceleration Spectral Density’) vs. frequency• Dimension is g2/Hz (or (m/s2)2xs)• PSD is the random vibration level, normalized with respect tothe bandwidth of analysis (since dealing with a broadbandexcitation)• Tabulated values will sometimes have a ‘gRMS’ value added:• This is the overall energy introduced by the randomvibration profile (integrates PSD vs. frequency)
Let’s mix things up – Sine on Random• Add discreet sinusoidal vibration tones to a broadbandrandom vibration background• Typical of helicopter and propeller aircraft applications(known blade passing frequencies on a randombackground)
Let’s mix things up – Sine on Random20 2000100 1000-41x10-31x10-21x10-11x1001x10Frequency (Hz)AccelerationSpectralDensity(G²/Hz)Acceleration Spectral DensityCh10 500 1000 1500 2000 2500 3000-10-50510Time (ms)Acceleration(G)Acceleration WaveformCh1
Let’s mix things up – Random on Random• Add narrowband random content onto a broadbandrandom background• Typical of tracked vehicles (military)• Narrowband content is swept across a frequency rangeto reflect speed changes
Let’s mix things up – Random on Random20 2000100 1000-41x10-31x10-21x10-11x1001x10Frequency (Hz)AccelerationSpectralDensity(G²/Hz)Acceleration Spectral DensityDemandCh10 500 1000 1500 2000 2500 3000 3500 4000-20-15-10-505101520Time (ms)Acceleration(G)Acceleration WaveformCh1
Random vibration vs. Transients…• Traditional random vibration averages out the actualvibration history.• Transients such as bumps, potholes, etc. do not translatewell into random• Another technique is needed to replicate transients
• Field data replication• Data collected in the field (acceleration vs. time)is directly played back and repeated in the labWhat are the different types of vibrations?
How are vibrations replicated in the lab?• Shakers• Mechanical• Electrodynamic• Hydraulic• Single/Multi-axis
How are vibrations replicated in the lab?• Mechanical shakers (‘direct drive’)• 1950’s• Cheap, sinusoidal excitation• Still used today for shipping vibration testCourtesy of Lansmont Corporation
How are vibrations replicated in the lab?• Electrodynamic shakers• Make most of the fleet of commercial test labs• Sequential, single-axis excitation• Big, highly-controlled ‘loudspeakers’• Come in various sizes and shapes (sliptable)• Rated in lbf• Payload and severity counterbalance eachother