page 174

13.5.2 Tachometers

•These devices measure the angular velocity of a rotating shaft.

•One way is to connect a DC generator (motor). The faster the shaft turns, the higher the voltage.

•Another technique uses a magnet with a pickup coil. As the magnet passes the coil a pulse is generated. The pulse magnitude and frequency are proportional to speed.

13.6 ACCELERATION

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

•

13.6.1 Accelerometers

Sensor

			Source of vibrations,
			or site for vibration
pre-
			measurement
amp

signal processor/ recorder

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

page 175

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.

Sealant to prevent moisture

hookup wire

	isolated		isolated
	stud	accelerometer
			wafer

surface

•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-vibrat- ing’ 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.