Velocity Transducer
Velocity transducer is also called siesmic pickup.
The relative motion between the permanent magnet and the coil generates a voltage that is proportional to the velocity of the motion.
The velocity transducer has an internal natural frequency of about 8 hz.
The velocity transducer is rather large.On small devices this added mass can significantly affect the vibration output.
The coil in the velocity pickup is sensitive to external electromagnetic fields.
Acceleration Transducers
The most common acceleration transducer is the piezoelectric accelerometer
It consist of quartz crystal with a mass bolted on top and a spring compressing the quartz.
A property of piezoelectric material is that it generates an electrical charge output when it is compressed.
The charge output is proportional to force F=ma,force is also proportional to acceleration.
Typically accelerometer has very high natural frequency,typically 25000 Hz.Its response is linear for about 1/3 of this range.
It has a useful frequency range of from about 5 to approximately 100000 Hz depending on its size.
The primary considerations in selecting an acceleromter are sensitivity and frequency response.
If high-amplitude motions are to be measured,i.e. greater than 10g,such as in shock measurement,then a low-sensitivity accelerometer is appropriate 10 mV/g or less.
If the level motion is to to be measured,such as building or structural motions at low frequencies then a high sensitivity accelerometer should be chosen 1000 mv/g.
For most machinery monitoring,100 mV/g sensitivity accelerometer provide the right balance of sensitivity and frequency response
Other considerations in accelerometer selection or transducer are
Temperature exposure
Linearity - It is expressed as the the percent deviation from a constant value of the sensitivity
Transverse Sensitivity is the ability of the transducer to detect motion in directions perpendicular to its sensitive axis.
Damping is very low in piezoelectric accelerometer but can be significant in other types,such as piezoresistive accelerometer
Strain sensitivity is the ability of the transducer to generate a signal when the base is distorted,such as when it is clamped against a nonflat surface.