Magnetic Sensors

Vibration Sensors Introduction / Selection
Characteristic
Flat Frequency Response
20–1,500 Hz
2–5,000 Hz
Phase Fidelity
2–5,000Hz
Reduced Noise at
Higher Frequencies
Linearity
Mounting in Any Orientation
Temperature Limitation
EMI* Resistance
Mechanical Durability

Coil and Magnet
Velocity Sensor

Piezoelectric
Velocity Sensor

Yes
No

Yes
Yes

Acceptable

Excellent

No
Good
Sensor Dependent
> +707°F (+375°C)
Acceptable
Good

Yes
Good
Yes
+248°F (+120°C)
Excellent
Excellent

*EMI–Electro Magnetic Interference

Traditional velocity sensors are of a mechanical design that uses an electromagnetic (coil and magnet) system to generate the velocity signal. Recently, hardier piezoelectric velocity sensors (internally integrated accelerometers) have gained in popularity due to their improved capabilities and more rugged and smaller size design. A comparison between the traditional coil and magnetic velocity sensor and the modern piezoelectric velocity sensor is shown in Table 1.
The electromagnetic (Inductive) velocity sensor does have a critical place in the proper sensor selection. Because of its high temperature capability it finds wide application in gas turbine monitoring and is the sensor of choice by many of the major gas turbine manufacturers.
The high temperature problems for systems using accelerometers can also be solved by splitting sensor and electronics (charge amplifiers). The sensor can have high temperature ranges up to +1,112°F (+600°C).
Some methods of investigating bearing defects and gear problems may require a higher frequency range and because the signals are generated by impact, the sensitivity should be lower. By the same means if the user is using the SKF “Enveloping Technique” then just the opposite is applicable.
ACCELEROMETERS
Piezoelectric accelerometers having a constant signal over a wide frequency range, up to 20 kHz's, for a given mechanical acceleration level, are