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Vibration sensors: three-pronged attack

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THE amplitude of vibration on any rotating or reciprocating equipment can be measured in three ways: acceleration, velocity or displacement.

Accelerometers

AN accelerometer is a sensor for converting acceleration to an electrical signal. Two common types are piezo-resistive and piezo-electric.

Acceleration is usually measured by sensing the forces generated by a small test mass attached to a sensing element. In some cases, the sensing element alone is used as the test mass.

Remember, in Newton’s Second Law of Motion (A=F/M or F=MA), the force on a body is proportional to its mass and its acceleration.

Thus the greater the mass (of the object being accelerated), the greater the amount of force needed (to accelerate the object). By measuring the force and knowing the mass, we can determine its acceleration.

PIEZO-ELECTRIC EFFECT

Many polymers, ceramics (lead-zirconate titanate [PZT]) and molecules such as water are permanently polarised: some parts of the molecule are positively charged, while other parts of the molecule are negatively charged.

When an electric field is applied to these materials, these polarised molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material.

Permanently polarised material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are called piezo-electric, and this phenomenon is known as the piezo-electric effect.

Conversely, an applied electric field can cause a piezo-electric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezo-electric effect.

More recently, we have seen the development of MEMS (micro-electro-mechanical systems) sensors such as the type made by Analog Devices. The most common application of MEMS is the automobile airbag. It is rarely used in machinery vibration monitoring applications mainly because of its poor noise characteristics.

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Some other problems are low bandwidth (<6 kHz) and low dynamic range (typically <50 G).

Although more big players (such as Motorola) are adapting this application, research and development continues in this area. The goal is to move these into new applications such as shock-detecting shipping monitors and tilt indicators.

Accelerometers (like Hardy Instruments’ HI-113) contain a PZT element, a small mass and a miniature amplifier. Hardy Instruments’ HS5701 also has a PZT, amplifier, integrator, filter and 4-20 mA drive circuit.

Velocity sensors

VELOCITY can be measured directly using a transducer that produces a signal proportional to the velocity of the sensor, or by mathematically converting an acceleration signal into a velocity reading.

The most common velocity sensors work like a generator, where a magnet moves within a coil to generate a voltage that is proportional to the velocity of the vibration. (CEC is a major manufacturer of this type of velocity sensor).

Coil-based velocity sensors are large mechanical devices and do not have the reliability or frequency range of the PZT accelerometers.

They are typically not recommended for new monitoring applications and you may see them in the field, or as spares for older systems. Before the advent of the HI 150 accelerometer, the coil-based velocity sensor’s one advantage was that it could operate up to 482 degrees C (such as in turbines). PZT devices cannot be operated above 150 or so degrees C. The HI 150 uses a new piezoelectric material that can operate at extreme temperatures.

With the advent in this new technology and the heightened focus on extending MTBF (Mean Time Between Failure), the old coil-based sensors are becoming much less applicable in modern day monitoring applications.

Displacement sensors

DISPLACEMENT sensors are used for shaft out-of-round, bearing wear and other applications that involve measuring distance.

They are also used in the Hardy rod-drop monitors to measure piston movement. Displacement measuring sensors measure the distance moved when “shaken”. The primary sensors in this area are eddy current displacement probes and laser devices.

EDDY CURRENT DISPLACEMENT PROBES

In machinery protection, the most common sensor for displacement is the eddy current probe.

The probe is built by winding a coil around a shaft. The coil is “driven” with a 2.5 Mhz signal (frequency depends on penetration depth in target material). The coil is mated to a probe driver by a cable. The probe driver contains electronics that measure the electrical characteristics of the coil.

As the probe is moved toward the metallic target, the target has currents generated in it from the field of the probe.

These currents then generate a reverse field, which changes the electrical characteristics of the coil.

The degree to which it affects the coil depends on the distance.

The electrical circuit in the driver measures these “coil changes” and converts them to an electrical signal. This gives us a voltage proportional to the displacement.

LASER DEVICES

Laser-based devices are primarily used for lab applications. They could be used in some probe applications, but probes have the advantage of working through oil and grime, where laser-based devices need an unobstructed view of the target exists.

PORTABLE SHAKERS

Hardy Instruments manufactures three “portable shakers”. These are designed for use in the field or workshop to verify the performance of vibration transducers, connectors, cabling, instruments or permanently installed machine vibration condition monitoring systems or indicators.

Shakers are battery powered devices that have a built-in reference accelerometer and indicator with calibration traceable to the US National Institute of Standards & Technology (NIST).

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