What are vibration transducers?
Vibration transducers used for machinery evaluations are devices that measure displacement of a shaft or motion of a structure and convert this motion into electrical signals. There are three basic types of vibration transducers:
- Accelerometers
- Velocity transducers
- Non-contact Displacement transducers
Features of vibration transducers
Of these three types of transducers, both accelerometers and velocity transducers can be either portable or permanently mounted and measure absolute motion. Non-contact displacement transducers measure relative motion and are permanently mounted. Relative motion using a non-contact displacement transducer represents the measurement of actual shaft movement while absolute motion represents movement of the bearing housing at the location of the transducer mounting.
Accelerometer
Accelerometers are the most commonly used type of transducers partly due to their low cost, wide usable frequency range, and their suitability in detecting rolling element bearing frequencies. With the wide range of machine types to be measured and the specific requirements in vibration detection within these machines, accelerometers must be properly selected for the type of measurement.
Accelerometers may be of the compression type which require a longer settling time and can be affected by thermal transients. Shear type accelerometers are more common and may be better suited for portability due to their quicker settling time. MEM sensors are used in a wide range of applications other than machine vibration detection but are making headway into the machinery vibration market as a low cost, but sometimes compromising, alternative to traditional sensors. Accelerometers may be wired, wireless, single axis, dual axis, dual axis with temperature, or triaxial.
How to Choose an Accelerometer
When selecting an accelerometer for a specific application you must first have knowledge of the machine to be tested. It is important to have knowledge of the:
Frequency Response
You will need to know the lowest frequency to be measured as well as the highest frequency to be measured.
Temperature Range
You will need to know the temperature limits of the sensor’s mounted environment.
Sensitivity
Accelerometer sensitivity is the ratio of the sensor’s electrical output to the mechanical input. Typical selections might include 10 mv/g for high frequency measurements, 100 mv/g for medium frequency measurements (the most common used for general machine types), and 500 mv/g and above for low frequency applications.
Dynamic Range
Dynamic Range of a vibration sensor is the comparison between the smallest signal it will sense without noise contamination to the largest signal (maximum g’s) it will accept without overloading. Dynamic Range is an important consideration not only for the separation of both high and low amplitude spectral content, but also on slow speed and very smooth operating machines. It is with these low vibration amplitude machines that Dynamic Range provides a means for low impact occurrences, such as bearing tones, to rise out of the noise floor.
What sensor does my machine need?
Most common machine types such as motors, pumps, fans, and some gearboxes are suitably surveyed using a 100 mv/g sensor. But when in doubt, sensor specific information may be found in the manufacturer’s specifications.
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