A Hall Effect sensor is a device that detects the presence of magnetic field. It is based on the Hall Effect. The Hall Effect was discovered by Edwin hall in 1869. When current is passed through the conductor and the same conductor is placed in magnetic field perpendicular to the current flow then a voltage called the hall voltage is generated perpendicular to both the current and magnetic field. This is known as Hall Effect.
The advantages of Hall effect sensors are
Non contact operation so there is no wear and friction. Hence unlimited number of operating cycles
High speed operation - over 100 kHz possible. Where as at high frequencies the inductive or capacitive sensor output begins to distort.
Can measure zero speed
Wide temperature range
Capable of measuring large current