THE USE OF FILTERING
Filters are used to observe only the frequencies which are of immediate interest.
VIBRATION INSTRUMENTATION
The most common transducers in vibration monitoring are
● Displacement transducer
● Velocity transducer
● Acceleration transducer
● Key phasor
Displacement Transducer (Proximity Probe)
The displacement transducer is commonly known as the proximity probe. An electromagnet is placed in the tip of the proximity probe. It generates an electric eddy current proportional to the gap between the probe and the metallic object to be measured.
The proximity probe (unlike velocity and acceleration transducers) must be mounted on a stationary support relative to the observed object. The proximity probes are useful for monitoring shaft orbits because they do not have to contact the moving surfaces.
Velocity Transducer
The velocity transducer is a movable permanent-magnet core with an enclosed coil. The relative motion between the magnet and the coil creates a voltage change which is proportional to velocity.
Figure 23.6 illustrates two velocity transducers. The displacement can be obtained by integrating the output of the transducer.
Acceleration Transducer
Most accelerometers use the characteristic of piezoelectric crystals to measure acceleration. By applying a force to two faces of the crystal, a voltage is generated by the crystal that is proportional to the force. The acceleration levels can be integrated to obtain velocity and integrated again to obtain displacement.
Accelerometers are used at higher frequencies more than velocity transducers or proximity probes.
Transducer Selection
The amount of energy dissipated by a vibrating machine is proportional to the displacement, velocity, and acceleration. They all must be considered. If the machine is exhibiting high acceleration at high frequency, the associated displacement is probably very low. If low acceleration was observed at low frequency, the associated displacement could be very high. Figure 23.7 illustrates the relationship between acceleration, velocity, and displacement measurements.
If the velocity of the vibrating machine remains constant over a frequency range, the acceleration will increase with frequency whereas the displacement will decrease with fre- quency. Therefore, the accelerometer would indicate large values at high frequencies and very low values at low frequencies. A displacement transducer would indicate large values at low frequencies and very low values at high frequencies.
In general, displacement transducers are useful from zero to several hundred hertz (10,000 r/min). Velocity transducers are useful from 5 to 2000 Hz (300 to 120,000 r/min). Accelerometers are useful from 1 Hz (60 r/min) to more than 50 kHz (3,000,000 r/min).
The accelerometer is the most versatile transducer available. However, it may not have adequate sensitivity at low frequencies. Also, the accelerometer may be oversensitive at high frequency. These problems can be corrected by selecting the proper transducers and by the addition of external filters to eliminate the high-frequency response of the accelerometer and enhance the sensitivity at low frequency.