When conducting vibration testing, an engineer needs to have not only a quality controller to process the data, but also 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).
In the vibration industry the three most common transducers that might be used in a vibration test are the acceleration sensor, the velocity sensor, and the displacement sensor. An examination of the advantages/disadvantages of each of these is described below.
Advantages/Disadvantages of Select Transducers
The three common transducers used in a vibration test are the displacement sensor, the velocity sensor, and the accelerometer (acceleration sensor). Each one has its unique place in vibration testing.
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 with hydraulic shaker systems. However, at higher frequencies the displacements are so tiny, the displacement sensors are ineffective at detecting the change in position.
Velocity Sensor
Velocity sensors measure the relative linear speed or angular speed of an object. A velocity sensor is effective in a mid-range of frequencies. Like an accelerometer, it is not effective in frequencies below 10 Hz. It is also unable to give effective data for frequencies of vibrations above 2 kHz. So 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 better choices.
Accelerometer
Accelerometers are the most commonly used and the best option for most vibration tests. They are effective over a very wide range of frequencies (3 Hz to 20 kHz). Accelerometers tend to be small instruments with a rigid construction – characteristics that permit 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 large dynamic range allowing test engineers to identify very large resonances for a specific UUT (unit under test). The main disadvantage of an accelerometer is its limitations at low frequencies (below 3 Hz).
Transducer Summary Table
|
Transducer |
Advantage |
Disadvantage |
|
Displacement Sensor
|
· Good at low frequency |
Measures relative position not absolute position Ineffective at high frequency due to small displacements |
|
Velocity Sensor
|
Good at middle frequency Laser sensors don't add mass |
Not effective below 10 Hz Not effective above 2000 Hz |
|
Acceleration Sensor
|
Good at middle frequency Good at high frequency Can produce velocity and displacement data via integration functions Small, rugged construction: useful in variety of environments |
· Add mass
|
If you found this blog post useful, download our handy checklist that will help you to run efficient vibration tests everytime!
Blog Post Resources:
National Instruments. “Vibration Signals and Transducers” (obtained on-line June 7, 2016 at http://zone.ni.com/reference/en-XX/help/372416A-01/svtconcepts/vbr_sig/).
Norton, Henry. Handbook of Transducers. Prentice-Hall, 1989. (obtained on-line June 8, 2016 at https://www.scribd.com/doc/39878089/Handbook-of-Transducers-Harry-N-Norton).
Phuc, Duong. Vibration Sensors 2012. Published June 11, 2015. (obtained on-line June 8, 2016 at http://www.slideshare.net/phucduong397/vibration-sensors-2012 ).
Tustin, Wayne. Random Vibration & Shock Testing. Equipment Reliability Institute, 2005.
