Magnetic Pick-up sensors

Magnetic pickup sensors are used to measure the speed of rotating objects such as shafts, flywheels, etc.  These sensors generate a sinusoidal waveform with a variable frequency.  The frequency of the signal depends on the speed of the rotating object. 

The magnetic pickup sensor works by the changing magnetic field when a metallic gear passes through it. The sensor consists of a permanent magnet and a coil mounted on the same axis.  Under static condition, the flux from the magnet flows from the north pole to the south pole.  The flux passes through the coil.  However, no voltage is produced as there is no relative motion between the magnet and the coil as they are fixed.  When the tooth of a gear to be measured passes through the sensor.

The magnetic flux lines are distorted as the tooth comes in front of the sensor. And when the tooth passes, the flux lines return to their original position.  They change again when the next tooth comes in front of the sensor.  This change in the flux induces a voltage in the coil placed in the sensor.  The frequency of this emf is dependent on the speed of the gear teeth. 

Thus by measuring the frequency, the speed of the rotating object is measured.   Magnetic pickups can