1.5.1 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

 

 

 

1.5.2 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.