Linear Velocity Transducer

 

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LVTs are kinds of speed sensors which are usually used to measure the speed of small motions.

 Linear Velocity Transducers use inductive technology. Moving a magnet through a coil of wire will      induce a voltage in the coil according to Faraday's and Lenz' Laws. This voltage is proportional to the  magnet's velocity and field strength. LVTs use this principle of magnetic induction, with a permanent  magnet and a fixed geometry coil, so the output voltage of the coil is directly proportional to the magnet's relative velocity over its working range.
During operation in the working range of the transducer, both ends of the magnet are inside the coil. With a single coil this would give zero output because the voltage generated by one pole of the magnet would cancel the voltage generated by the other pole. To avoid this, the coil is divided into two sections, so the N (North) pole of the magnet will induce a voltage in one coil and the S (South) pole will induce a voltage in the other coil. These two coils are then connected in series-aiding, to obtain a DC voltage output proportional to the magnet's velocity.
 

 

 

Coils in Series

For series operation, the blue and green leads are tied together and the output is taken from the black and red leads. With the coils wired in series, the output is summed, producing the maximum sensitivity. Besides a high sensitivity, the transducer exhibits excellent noise immunity, attributed to the coils being wound in series opposition. Noise generated on one coil will be equal in magnitude but opposite in polarity to the other coil.

 

 

Coils in Parallel

Parallel operation is achieved by tying the black and blue leads together, and the green and red leads together. The two connections create the terminals for the output. This configuration cuts the sensitivity in half, and reduces the source impedance by a factor of 4. The benefits of this arrangement are: lower output, for use in high speed applications; lower output impedance, for compatibility with electronics with a low input impedance, and higher frequency response for a given load impedance.

 

 Major manufacturers of LVTs are Trans-Tek Inc. and GE Company.
 The working displacement ranges are from 0.5 in. to 24 in., and typical sensitivities are from 40 mV/ips (inches per second) to  600 mV/ips.

The series100 LVTs are the most popular ones; produced by Trans-Tek .The Series 100 Linear Velocity Transducers are inductive, instantaneous   velocity transducers. By moving a magnet through a pair of series opposed coils, the transducer will output a DC voltage proportional to the instantaneous velocity. No excitation voltage is required. Available ranges are from 0.5" to 24.0".
You can review Series100's data sheet here.

 Main Features of series100 :

 Measurement Ranges               :      0.5 to 24 inches
 Maximum Non-Linearity           :      2.5% Full Scale
 Housing Outer Diameter          :      varies
 Operating Temperature Range :      -65F to +140F
 Sensitivity                             :       Up to 500 mV/inch/second
 Excitation Voltage                  :        None required

Two other models of LVTs, produced by GE company are models 9200 and 74712.For information in details, review them on their data sheet.


 

Some of other manufacturers are Robinson-Halpern Products and Macro sensors.

 



 

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