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page 173

13.4.4 Interferometers

Uses the fact that when light is 180 degrees out of phase it cancels out.

By finding the phase between outgoing and returning light (from lasers typically) the distance can be found.

13.5 VELOCITY

This can be measured with dedicated sensors.

We may also differential the output of a position sensor to find a velocity

13.5.1 Velocity Pickups

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

hookup wire

stud

silicone

velocity pickup

grease

 

hookup wire

velocity pickup

magnet

surface

surface

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