Vibration Transducer
Vibration Transducer
Vibration Transducer
ON
VIBRATION TRANSDUCERS
VIBRATION TRANSDUCERS
Transducer:-
1) Velocity pick-up:-
(Moving coil type)
Construction:-
The system consists of a coil of fine wire supported by soft springs. A
permanent magnet, firmly attached to the case of the transducer,
provides a strong magnetic field, around the coil.
Theory of Operation:-
Whenever this transducer is fixed or held tightly against a vibrating
object, the permanent magnet vibrates, while the spring suspended coil
of wire remains stationary in space. When a coil of wire cuts magnetic
lines of force, a voltage is generated in that wire. The voltage is
proportional to the velocity of motion, the strength of the magnetic
field and no. of turns of wire in the coil. As the last two are constant,
therefore, the voltage generated is directly proportional to velocity of
motion (vibration).The voltage generated is transmitted by cable to a
vibration meter, Analyzer or Data logger.
Advantage:-
1) Velocity pick-ups are easy to apply and can be hand held.
2) They are self generating and require no power inputs.
3) They have relatively high electrical output and system calibration is
virtually unaffected by cable length.
Limitations:-
1) Velocity pick-ups are in general larger and weigh more than other
pick-ups.
2) Velocity pick-ups can only be used under a certain maximum and
minimum range of vibration amplitudes and frequencies.
3) At low frequencies of vibration, the sensitivity of velocity pick-up
decreases because the coil no longer remains fixed in space but tends
to follow the motion of the magnet.
4) The amplitude readings below 600 CPM for the standard velocity
transducers are not true readings.
5) The velocity transducer uses a fixed, internal magnetic field. Any
distortion of this magnetic field(like measuring the vibration on large
A.c motors or Alternators) interfere with the accuracy of the
measurement.
Solution:-It is recommended to install the pick-up within a magnetic
shield and the temporary solution is to use a long extension probe.
2) Accelerometers:-
Construction:-
The seismic mass is clamped to the base by an axial bolt bearing
down on a circular spring. The piezo-electric element is squeezed
between the mass and the base. When a piezo-electric material
experiences a force, it generates an electric charge between its
surfaces. There are many such materials, with quartz being one of the
most commonly used.
Theory of operation:-
when accelerometer is fixed or held against a piece of vibrating
machinery, the mechanical vibrations are passed through the frame to
a piezo-electric material. The material has the ability to generate an
electrical charge in response to a mechanical force applied to it.
Electrical charge is proportional to the amount of vibration
acceleration. The electrical charge generated by the piezo-electric
material is very small so high gain electronic amplifier is used to
amplify the value.
Advantage:-
1) Accelerometers are small, light weight rugged transducers.
2) They operate over frequency and temperature ranges and withstand
high vibration levels.
3) Accelerometers are much less sensitive to stray magnetic fields than
velocity transducers.
4) Good for machines operating at very high rotating speeds.
Limitations:-
1) Accelerometers are more susceptible to ground loops, radio
frequency interference and dirty or poor contacts.
2) Cable length effects may reduce sensitivity.
Construction:-
Non-contact transducers are installed in the machine with the tip of the
transducer in close proximity to the rotating shaft. This distance
between the tip of the transducer and shaft is called the gap. These
transducers can be used with portable meters and analyzers as well as
permanently mounted monitors. It consists of a sensor head, coaxial
cable, an amplifier, an amplitude detector and an output circuit.
Theory of operation:-
Unlike velocity and acceleration transducers. However, a non-contact
transducer does not have an element that generates a voltage for
electrical charge in response to vibration. A non-contact sensor
requires external electronic circuitry to generate a very high frequency
Ac signal and detect vibration in the Ac signal caused by the vibration
of the shaft. As the shaft moves relative to the tip of the pick-up, the
strength of the electrical signal changes proportional to the movement.
This variation in the strength is proportional to the amount of
vibration. This displacement signal is sent through a cable to a
vibration meter, monitor or analyzer. As the gap decreases, load and
strength of signal increases.