Vibration Characterization

Vibration Characterization involves documenting the offending disturbances in the time domain and/or frequency domain. Steady vibration disturbances are often best characterized by the frequency components of the disturbance. Transient disturbances are often characterized in the Time Domain. We analyze the disturbance in both domains using a variety of signal processing and analysis hardware and software, however our most valuable resource is our experience and judgment.

Semiconductor AFM Tool Response to Environment

Fatigue Cracking on Turbine foundation

Note that we very often make measurements of different signal types, such as vibration, acoustic noise, pressure, strain, air flow, current, voltage, etc. Making a measurement that is actually useful can be challenging and depends not only on the quality of the measurement, but also on an understanding of the particular structural dynamics at hand.

For instance, our initial evaluation of the dynamics influences the choice of measurement location, sensor, sensor mounting, length of capture, bandwidth, windowing, and an idea of what to expect and how to interpret it. We provide this understanding as part of our vibration testing services to quickly and efficiently characterize the root of the vibration problem and the dynamics at play.

Shipping Vibration Characterization

for Hard Disk Rack

Hard Disk Rack Vibration Test Rolling Over Bumps

In the real world there are judgments that must be made as to where to mount sensors to capture a good representation of the vibration response. Often we are testing structures and products that are not complete, partially built and missing components. This is because the working products are often in use. We look at the working equipment, structures, products, consider the mass distribution, stiffness peculiarities, how pieces are put together on the working system, and come up with modifications to the test setup to better represent real structure.

Semiconductor Turbo Pump Characterization

Helicopter Instrument Vibration, Engine Mount Modifications

This is our Bread and Butter and we are good at it.

Here is a list of some common vibration characterization measurements:

  • Acceleration - Using single and tri-axial accelerometers of various form factors and sensitivities

  • Displacement - Using the filtered integral of acceleration

  • Relative displacement - Measured directly between critical components using a variety non-contact sensors, with sub-nanometer resolution

  • Velocity - Used for comparison with various industry standards, or velocity ripple on a scanning stage system, or 1/3 octave band floor rms floor velocity levels.

  • Rotation can be measured in a number of ways but it is often best to measure it directly.

  • Rotational velocity can also be measured in a number of ways but is also often best to used a sensor specifically designed to measure rotational velocity.

  • Strain - Used to identify force transmission path, structural integrity, as a sensor for dynamic measurement.

  • Pressure

  • Flow

  • Force

  • Magnetic Field

  • Light Intensity

  • Any signal

We consider the expected frequency range and select our sensors and their mounting dynamics appropriately. Often we don't know what to expect exactly until we are on site looking at the disturbance spectra and waveform. We then have the judgment and experience to make modifications to our test setup and instrumentation to get meaningful measurements that will answer the pertinent questions being asked.

Thermal Performance and Vibration Optimization

Spectra of Vibration on Turbine Foundation

 

Time Domain Analysis

Short duration disturbances are often at the heart of a system problem. We call these short duration events transients. They may involve the system ringdown to a motor startup, or a robot move. The transient may be a velocity ripple that shows up as a disturbance in part of an inspection scan, or the slipping and sliding of a product due to high-G bumps in automated handling. These events must be measured and characterized principally in the time domain to make headway in understanding the problem at hand.

Transient disturbances present more of a challenge than do disturbances that are steady in time. Short duration disturbances are often not well characterized in the frequency domain using FFT analysis techniques because the details of the wave form are what is most important. These details are lost in the magnitude of the FFT. Transients are also often hidden under layers of system vibration and instrumentation noise.

Time Domain Accelerations Supporting Samples During Automated Moves

Yet transient disturbances often have patterns to them that are important. For example, the timing of horizontal to vertical accelerations are critical to evaluating when a part may overcome friction and slip.

Horizontal vs Vertical Acceleration Plot to Estimate When Slip May Occur

The time domain waveform often holds a lot of information that can be used to form a hypothesis for a vibration problem. Below is a picture of a waveform measured on a part that was failing and impacting its supporting frame. At the start of the project our thinking was that the steady operational vibration was suspect. However, we found that, due to a very soft isolation system, the part appeared to impact the frame resulting in very high levels of inertial force. Fatigue often occurs with either many cycles of stress just above the SN curve, or a smaller number of cycles well above the SN curve. Our testing showed that the latter case was most probable and a new mounting system was proposed.

Acceleration Measurement of Non-Linear Impacts During Product Testing to Diagnose Failure Mode

Understanding the instrumentation and the effective time-constants of the instrumentation response is also very important, as is the awareness of non-linear behaviors that include amplitude and frequency dependence, motion history dependence, direction and time dependence.

 

We have developed techniques to get past these challenges and get at the heart of the problem. To do this well requires a full understanding of the physics of the sensors and the system under test. Give us a call and we can discuss your unique situation in more detail. 

Contact Us:
Call Reuben Hale, P.E. at 510-507-1300
or click this email link:
Reuben@ResponseDynamics.com
or submit your info below:

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