VIBRATION MEASURING INSTRUMENTS
A.) VIBRATION MEASURING INSTRUMENTS
–Analyzer : Measures all the three characteristics viz. Amplitude, Freq. and Phase
–Meter : Measures only Amplitude
Thus we can have METERS of various types
Displacement And Velocity- Normally uses velocity pickup
Displacement, Velocity, Acceleration and Bearing Condition- Normally uses Accelerometer
The heart of the instrument is the sensor
B.) Vibration Sensors
- Acceleration Sensor – Accelerometer
- Velocity sensor – Velocity sensor or seismic sensor
- Displacement sensor – Non Contact pickup
1.) Accelerometer
- Uses piezoelectric crystal
- Normally uses internal amplifier for boosting signal
- It will have low temp application up to 150 degree C
- For high temperature application amplifier is not given inside
- This causes low signal strength and we use gold plated contacts and small cable lengths (output rated in pc per g). The amplification is done outside
- Normal accelerometer (amplifier inside) will have strong signal output and can sustain cable lengths of 300 meter
- Available in several design, can go from DC to 10k Hz freq (600,000 CPM)
- Can measure Acceleration, velocity and displacement
- It is advised to involve suppliers or persons knowledgeable in this area to choose accelerometers for on line monitoring applications
2.) Velocity sensor
- This is self generating (764 mv per mm/sec)
- Can sustain cable length of 1000 feet easily
- For longer length intermediate amplification is needed
- The temperature rating is higher up to 250 degree C (no electronics inside)
- Sensitive to 60,000 CPM
- Can check velocity and displacement only.
3.) Non contact type
C.) ABSOLUTE BEARING VIBRATION
VELOCITY TRANSDUCER
SEISMIC MASS TYPE
Works on PLUNGER COIL PRINCIPLE.
Two permanent magnets rigidly fixed to the casing with coils arranged as seismic mass.
The Relative motion between the coil and magnet generates voltage proportional to vibration velocity.
D.) ABSOLUTE SHAFT VIBRATION
- RELATIVE SHAFT VIBRATION IS BASED ON EDDY CURRENT PRINCIPLE.
- PROXIMOTY PROBE SENSES THE DISTANCE BETWEEN THE PROBE TIP AND THE OBSERVED SURFACE.
- PROXIMETER GENERATES A FREQUENCY SIGNAL, WHICH IS RADIATED THROUGH THE PROBE TIP INTO THE OBSERVED SURFACE. EDDY CURRENTS ARE GENERATED IN THE SURFACE AND LOSS OF STRENGTH IN THE RETURN SIGNAL IS DETECTED BY THE PROXIMETER.
