ACCELERATION SENSORS (compression type)
Operating principle:A mass is mounted upon the piezo element (crystal or ceramic). Due to machine vibration the seismic mass deforms the piezo element (squeeze and release). The mechanical force of the seismic mass is proportional to the machine vibration (acceleration). The piezo crystal generates a proportional electrical charge. The charge amplifier converts the electrical charge into a voltage output.
Applications:
Absolute casing vibration
Bearing condition
In combination with a trigger measurement: Vector measurement (magnitude and phase)
Advantages:
Wide frequency and amplitude range
Rugged construction, small designs
Arbitrary measuring direction
Low sensitivity to magnetic fields
Drawbacks:
External power supply required
Low sensitivity at low frequencies (roll-off)
Limited operation temperature due to internal amplifier (<125 °C)
VELOCITY SENSORS
Operating principle:
The case of the sensor is mounted to the measured object. The magnet of the velocity sensor is suspended on a membrane (spring) which moves through a coil of wire. Due to the vibration, the coil of wire moves through the magnetic field of the permanent magnet and generates (induces) a voltage signal which is proportional to the vibration (velocity).
Application:
Absolute casing vibration
In combination with a trigger measurement: Vector measurement (magnitude and phase)
Advantages:
Rugged construction
High sensitivity at low frequencies
High output signal with low internal resistance
simple mounting
No external power supply needed
Drawbacks:
Upper frequency limit below 2 kHz
Resonance frequency at 8 Hz / 15 Hz; linearization required
Some types are mounted vertically, others horizontally
Relatively large
DISPLACEMENT SENSORS
Operating principle:
The displacement sensor oscillator generates a constant frequency sine wave that passes through an inductive coil, which produces an electromagnetic field in front of the sensor face. When a target metal object comes close to this field, some of the electromagnetic energy is transferred to the target as eddy currents. This transfer of energy reduces the amplitude of the oscillator, which is inversely proportional to the distance of the target metal object being monitored to the face of the sensor.
Applications:
Used on journal-bearing machines
Two sensors are used, at 90º to each other, to determine the position of the shaft (X-/Y-measurement).
Relative shaft vibration, axial vibration, speed/trigger
In combination with a trigger measurement: Vector measurement (magnitude and phase)
Differential Expansion
Rod-drop (on compressors)
Eccentricity
Advantages:
Usable with all conducting materials
Show the movement of the shaft within the bearing
Not influenced by oil or water
Low frequency response (to 0 Hz)
Drawbacks:
high installation effort (permanently mounted)
Calibration dependent on shaft material
Shaft runout might produce false signals
TRANSMITTERS
They simply transfer (“transmit”) a pre-defined specific measurement (e.g. vibration velocity mm/s rms) and measurement range (e.g. 0…20 mm/s rms) to a DCS or PLC via a standardized signal like 4-20 mA, for example.
A transmitter thus “translates” a specific measurement type and range into a signal that a superior controlling system can process.