When conducting a vibration test, an engineer needs a quality controller to process the data as well as the appropriate transducers to collect the data. A transducer is an electronic piece of equipment that converts energy—or, more precisely, the signal carried in a particular form of energy—to another form of energy.
Broadly speaking, microphones, speaker systems, Geiger counters, strain gauges, and pressure sensors are examples of transducers. A handy reference manual on a variety of transducers is Henry Norton’s Handbook of Transducers (Prentice-Hall, 1989). The VRU course Sensors for Vibration Testing also discusses this topic more in-depth.
Advantages/Disadvantages of Select Transducers
In the vibration testing industry, the three most common transducers are the acceleration sensor, the velocity sensor, and the displacement sensor. An examination of the advantages/disadvantages of each of these is described below.
Displacement Sensor
Displacement sensors measure the relative position and change in position of an object. They are very effective and accurate at the low frequencies that are often found in hydraulic shaker systems. However, at higher frequencies, the displacements are small and the displacement sensors are ineffective at detecting the change in position.
Advantage
Good at low frequencies
Disadvantages
Measures relative position, not absolute position
Ineffective at high frequency due to small displacements
Velocity Sensor
Velocity sensors measure the relative linear speed or angular speed of an object. A velocity sensor is effective for mid-range frequencies. Similar to an accelerometer, a velocity sensor is not effective for frequencies below 10Hz. It is also unable to give effective data for frequencies of vibrations above 2kHz. Although there may be situations where a velocity sensor may be helpful, in most circumstances the accelerometer (for mid and high frequencies) or a displacement sensor (for low frequencies) would be a better choice.
Advantages
Good at middle frequency
Laser sensors don’t add mass
Disadvantages
Not effective below 10Hz
Not effective above 2,000Hz
Accelerometer
Accelerometers are the most commonly used and best option for most vibration tests. They are effective over a wide range of frequencies (3Hz to 20kHz). Accelerometers tend to be small instruments with rigid construction, which permits them to be useful in the most hostile of environments. Velocity and displacement data can easily be obtained from the acceleration data through integration processes. In addition, accelerometers have a large dynamic range, which allows test engineers to identify large resonances for a specific device under test (DUT). The main disadvantage of an accelerometer is its limitations at low frequencies (below 3Hz).
Advantages
Good at middle frequency
Good at high frequency
Can produce velocity and displacement data via integration functions
Small, rugged construction: useful in a variety of environments
Disadvantages
Add mass
One of the challenges in vibration testing is mass loading. The resonant frequency of a vibrating DUT will change depending on whether the technician adds an additional mass, such as an accelerometer. The paper, “Mass Loading and Benefits of a Laser Vibrometer” explains this challenge and a solution.
If you found this blog post useful, download our handy checklist to run an efficient vibration test every time!