Vibration Pickup

A vibration pickup is an electromechanical transducer -- such as a vibration sensor -- capable of converting mechanical vibrations into electrical voltages. Depending upon their sensing element and output characteristics, such pickups are referred to as accelerometers, velocity pickups, or displacement pickups.

The accelerometer consists essentially of a mass which is seismically supported with respect to a surrounding case by means of a spring and guided to prevent motions other than those along the seismic direction of support. The mass exerts a force on the spring's support which is directly proportional to the acceleration being measured. This, in turn, is converted into an electrical voltage by means of stresses produced in a piezoelectric crystal.

The velocity pickup generates a voltage proportional to the relative velocity between two principal elements of the pickup, the two elements usually being a coil of wire and a source of magnetic field (see Fig. 1).

Fig. 1: Velocity pickup. The coil swings on one end of an arm which is supported by bearings at the opposite end. The case follows the motion of the structure to which it's attached.

The displacement pickup is a device that generates an output voltage which is directly proportional to the relative displacement between two elements of the instrument. These pickups are similar in construction and behavior to velocity pickups. The only essential difference is the use of a frequency-weighting network, required to make them direct-reading.