29.6 VIBRATION TRANSDUCERS

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29.6.1 Velocity Pickups

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Output voltage is proportional to velocity (V/(cm/s))

These devices have low natural frequencies, and are used for signals with higher frequencies.

well suited to measuring severe vibrations, but it may be affected by noise from AC sources.

because signals are velocity, some form of integration must be done, making these devices bulky, and somewhat inaccurate

There are two common methods for mounting velocity pickups,

- Magnetic mounts allow fast and easy mounting, but the magnetic mount acts as a slight spring mass isolator, limiting the frequency range.

- Stud mounted transducers have a thin layer of silicone grease to improve contact

29.6.2 Accelerometers

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Compared to velocity pickups

- smaller

- more sensitive

- wider frequency range

electronic integrators can provide velocity and position

The accelerometer is mounted with electrically isolated studs and washers, so that the sensor may be grounded at the amplifier to reduce electrical noise.

Cables are fixed to the surface of the object close to the accelerometer, and are fixed to the surface as often as possible to prevent noise from the cable striking the surface.

Background vibrations in factories are measured by attaching control electrodes to `non-vibrating' surfaces. (The control vibrations should be less than 1/3 of the signal for the error to be less than 12%)

Piezoelectric accelerometers typically have parameters such as,

- -100to250°C operating range

- 1mV/g to 30V/g

- operate well below one forth of the natural frequency

Accelerometer designs vary, so the manufacturers specifications should be followed during application.

There is often a trade-off between wide frequency range and device sensitivity (high sensitivity requires greater mass)

Two type of accelerometers are compression and shear types.

Mass of the accelerometers should be less than a tenth of the measurement mass.

Accelerometers can be linear up to 50,000 to 100,000 m/s**2 or up to 1,000,000 m/s**2 for high shock designs.

Typically used for 10-10,000 Hz, but can be used up to 10KHz

Temperature variations can reduce the accuracy of the sensors.

typical parameters are,

These devices can be calibrated with shakers, for example a 1g shaker will hit a peak velocity of 9.81 m/s**2

29.6.3 Preamplifiers

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The input can be either current or voltage

sensor signals often have very low values.

the output of preamplifiers is typically voltage

these devices can also provide isolation both to and from the sensor

current amplifiers generally are more costly, but they are more immune to noise.

29.6.4 Modal Analysis

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Basically, excite a vibration, and measure how it is transmitted through a structure