13.6 ACCELERATION

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Like velocity we may also find acceleration by differentiating velocity, or by differentiating position twice.

13.6.1 Accelerometers

Vibration Source - hammers can be used to generate impulse/step function responses. Load cells/vibrators can be used to excite frequency responses (Bode plots and phase shift plots)

Sensors - Velocity Pickups/Accelerometers - lightweight devices that are mounted on structures. They produce small voltages (approx. 10mV). Velocity meters are not as accurate as accelerometers. Accelerometers are very common, and are used for vibrations above 1KHz. Many other sensors are possible.

Preamplifiers - Can power sensors, filter and amplify output.

Signal Processor - Many types used, from software packages, to older pen based plotters, or tape recorders

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