Vibration Sensors & Transmitters

Machine vibrations may take various forms. They vibrate quickly or slowly; over a short or long distance. Vibrations can be desired but usually, they are unintended and undesirable.As suppliers of machinery protection and condition monitoring systems, we at B&K Vibro deal with unintended and undesirable machine vibrations. To do this, we need sensors (also called transducers or probes). These typically measure vibration on machines like motors, pumps, fans, turbines, compressors, gearboxes and other reciprocating and oscillating machinery.Machine vibrations are measured in units of displacement, velocity or acceleration. There are relative and absolute vibration measurements, typically measured on shafts and bearing casings:

• Absolute vibration indicates the distance to free space
• Relative vibration indicates the distance relative to a fixed point on the machine

There are two basic measuring principles for vibration measurements:

• Non-contacting measuring methods (displacement). For these methods, eddy-current sensors and laser optical sensors are used.
• Contacting measuring methods (acceleration, velocity). For these methods, piezoelectric or piezoresistive sensors or inductive sensors are used.

There are 3 types of vibration sensors commonly used:

• Displacement sensors
• Velocity sensors (“velocimeters”)
• Acceleration sensors (“accelerometers”)

We select the sensor type/units based on:

• Frequencies of interest (higher or lower frequencies)
• Bearing type (rolling-element bearing or joural bearing)

Displacement measurement is the distance or amplitude displaced from a resting position. The measuring unit is µm or mils.

Velocity is the rate of change of displacement with respect to change in time. This is measured in mm/s or in/s.

Acceleration is the rate of change of velocity with respect to change over time and is measured in m/s² or g.

Sensors for machines need to meet these requirements:

• Rugged industrial design for usage in harsh environment
• Wide temperature range of -50 °C to +120 °C up to +350 °C
• High reliability
• Durability (15 to 20 years lifetime)
• Flexible mechanical design to adapt to the machine
• Usage in explosive and hazardous areas

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.

Quality is decisive for a sensor. No matter how sophisticated the signal processing capability of your monitoring system is, it can never compensate for an under-performing sensor. And we are not just talking about precision. It also has to be robust for the environment it is working in. An inexpensive sensor that unexpectedly breaks down is not just a nuisance, but it also poses a catastrophic risk for critical machinery if there is no monitoring. This applies to many balance-of-plant machines as well. Keep in mind that proper health awareness of your machines depends on quality sensors.We develop and manufacture the largest part of them at our headquarters in Darmstadt/Germany. More than 500,000 of our sensors are in use at customer installations worldwide.Use our filtering tool to select the sensor you require or get in touch if you need help. We’re happy to assist you. Our service team is available for any installation or retrofit issues.